Make sure bitmap encoder preserves 4 bits per pixel, when the input is 4 bit

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -1304,8 +1304,9 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             this.bmpMetadata = this.metadata.GetBmpMetadata();
             this.bmpMetadata.InfoHeaderType = infoHeaderType;
 
-            // We can only encode at these bit rates so far (1 bit and 4 bit are still missing).
-            if (bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel8)
+            // We can only encode at these bit rates so far (1 bit per pixel is still missing).
+            if (bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel4)
+                || bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel8)
                 || bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel16)
                 || bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel24)
                 || bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel32))

tests/ImageSharp.Tests/Formats/Bmp/BmpEncoderTests.cs

@@ -40,6 +40,8 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
         public static readonly TheoryData<string, BmpBitsPerPixel> BmpBitsPerPixelFiles =
         new TheoryData<string, BmpBitsPerPixel>
         {
+            { Bit4, BmpBitsPerPixel.Pixel4 },
+            { Bit8, BmpBitsPerPixel.Pixel8 },
             { Rgb16, BmpBitsPerPixel.Pixel16 },
             { Car, BmpBitsPerPixel.Pixel24 },
             { Bit32Rgb, BmpBitsPerPixel.Pixel32 }