diff --git a/src/Avalonia.Animation/Animatable.cs b/src/Avalonia.Animation/Animatable.cs
index 9e9b84537b..067d9f462f 100644
--- a/src/Avalonia.Animation/Animatable.cs
+++ b/src/Avalonia.Animation/Animatable.cs
@@ -93,17 +93,17 @@ namespace Avalonia.Animation
                 var oldTransitions = change.OldValue.GetValueOrDefault<Transitions>();
                 var newTransitions = change.NewValue.GetValueOrDefault<Transitions>();
 
-                if (oldTransitions is object)
-                {
-                    oldTransitions.CollectionChanged -= TransitionsCollectionChanged;
-                    RemoveTransitions(oldTransitions);
-                }
-
                 if (newTransitions is object)
                 {
                     newTransitions.CollectionChanged += TransitionsCollectionChanged;
                     AddTransitions(newTransitions);
                 }
+
+                if (oldTransitions is object)
+                {
+                    oldTransitions.CollectionChanged -= TransitionsCollectionChanged;
+                    RemoveTransitions(oldTransitions);
+                }
             }
             else if (_transitionsEnabled &&
                      Transitions is object &&
@@ -111,8 +111,10 @@ namespace Avalonia.Animation
                      !change.Property.IsDirect &&
                      change.Priority > BindingPriority.Animation)
             {
-                foreach (var transition in Transitions)
+                for (var i = Transitions.Count -1; i >= 0; --i)
                 {
+                    var transition = Transitions[i];
+
                     if (transition.Property == change.Property)
                     {
                         var state = _transitionState[transition];
diff --git a/src/Avalonia.Base/Data/Core/Plugins/InpcPropertyAccessorPlugin.cs b/src/Avalonia.Base/Data/Core/Plugins/InpcPropertyAccessorPlugin.cs
index f2ed86d2aa..84ef0fb695 100644
--- a/src/Avalonia.Base/Data/Core/Plugins/InpcPropertyAccessorPlugin.cs
+++ b/src/Avalonia.Base/Data/Core/Plugins/InpcPropertyAccessorPlugin.cs
@@ -1,7 +1,5 @@
 using System;
 using System.ComponentModel;
-using System.Linq;
-using System.Reactive.Linq;
 using System.Reflection;
 using Avalonia.Utilities;
 
@@ -31,7 +29,11 @@ namespace Avalonia.Data.Core.Plugins
             Contract.Requires<ArgumentNullException>(propertyName != null);
 
             reference.TryGetTarget(out object instance);
-            var p = instance.GetType().GetRuntimeProperties().FirstOrDefault(x => x.Name == propertyName);
+
+            const BindingFlags bindingFlags = BindingFlags.NonPublic | BindingFlags.Public |
+                                              BindingFlags.Static | BindingFlags.Instance;
+
+            var p = instance.GetType().GetProperty(propertyName, bindingFlags);
 
             if (p != null)
             {
diff --git a/tests/Avalonia.Animation.UnitTests/AnimatableTests.cs b/tests/Avalonia.Animation.UnitTests/AnimatableTests.cs
index b5c61883e7..784f40fe1f 100644
--- a/tests/Avalonia.Animation.UnitTests/AnimatableTests.cs
+++ b/tests/Avalonia.Animation.UnitTests/AnimatableTests.cs
@@ -2,6 +2,7 @@
 using Avalonia.Controls;
 using Avalonia.Data;
 using Avalonia.Layout;
+using Avalonia.Media;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Moq;
@@ -265,6 +266,70 @@ namespace Avalonia.Animation.UnitTests
             }
         }
 
+        [Fact]
+        public void Replacing_Transitions_During_Animation_Does_Not_Throw_KeyNotFound()
+        {
+            // Issue #4059
+            using (UnitTestApplication.Start(TestServices.RealStyler))
+            {
+                Border target;
+                var clock = new TestClock();
+                var root = new TestRoot
+                {
+                    Clock = clock,
+                    Styles =
+                    {
+                        new Style(x => x.OfType<Border>())
+                        {
+                            Setters =
+                            {
+                                new Setter(Border.TransitionsProperty,
+                                    new Transitions
+                                    {
+                                        new DoubleTransition
+                                        {
+                                            Property = Border.OpacityProperty,
+                                            Duration = TimeSpan.FromSeconds(1),
+                                        },
+                                    }),
+                            },
+                        },
+                        new Style(x => x.OfType<Border>().Class("foo"))
+                        {
+                            Setters =
+                            {
+                                new Setter(Border.TransitionsProperty,
+                                    new Transitions
+                                    {
+                                        new DoubleTransition
+                                        {
+                                            Property = Border.OpacityProperty,
+                                            Duration = TimeSpan.FromSeconds(1),
+                                        },
+                                    }),
+                                new Setter(Border.OpacityProperty, 0.0),
+                            },
+                        },
+                    },
+                    Child = target = new Border
+                    {
+                        Background = Brushes.Red,
+                    }
+                };
+
+                root.Measure(Size.Infinity);
+                root.Arrange(new Rect(root.DesiredSize));
+
+                target.Classes.Add("foo");
+                clock.Step(TimeSpan.FromSeconds(0));
+                clock.Step(TimeSpan.FromSeconds(0.5));
+
+                Assert.Equal(0.5, target.Opacity);
+
+                target.Classes.Remove("foo");
+            }
+        }
+
         private static Mock<ITransition> CreateTarget()
         {
             return CreateTransition(Visual.OpacityProperty);
diff --git a/tests/Avalonia.Benchmarks/Data/PropertyAccessorBenchmarks.cs b/tests/Avalonia.Benchmarks/Data/PropertyAccessorBenchmarks.cs
new file mode 100644
index 0000000000..5c2502422a
--- /dev/null
+++ b/tests/Avalonia.Benchmarks/Data/PropertyAccessorBenchmarks.cs
@@ -0,0 +1,57 @@
+using System;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+using Avalonia.Data.Core.Plugins;
+using BenchmarkDotNet.Attributes;
+using JetBrains.Annotations;
+
+namespace Avalonia.Benchmarks.Data
+{
+    [MemoryDiagnoser, InProcess]
+    public class PropertyAccessorBenchmarks
+    {
+        private readonly InpcPropertyAccessorPlugin _plugin = new InpcPropertyAccessorPlugin();
+        private readonly TestObject _targetStrongRef = new TestObject();
+        private readonly WeakReference<object> _targetWeakRef;
+
+        public PropertyAccessorBenchmarks()
+        {
+            _targetWeakRef = new WeakReference<object>(_targetStrongRef);
+        }
+
+        [Benchmark]
+        public void InpcAccessor()
+        {
+            _plugin.Start(_targetWeakRef, nameof(TestObject.Test));
+        }
+
+        private class TestObject : INotifyPropertyChanged
+        {
+            private string _test;
+
+            public string Test
+            {
+                get => _test;
+                set
+                {
+                    if (_test == value)
+                    {
+                        return;
+                    }
+
+                    _test = value;
+
+                    OnPropertyChanged();
+                }
+            }
+
+            public event PropertyChangedEventHandler PropertyChanged;
+
+            [NotifyPropertyChangedInvocator]
+            protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
+            {
+                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+            }
+        }
+    }
+}