Update Part-4.md

docs/en/Tutorials/Part-4.md

@@ -75,9 +75,12 @@ If you had created a data seed contributor as described in the [first part](Part
 Add a new test class, named `BookAppService_Tests` in the `Books` namespace (folder) of the `Acme.BookStore.Application.Tests` project:
 
 ````csharp
+using System;
+using System.Linq;
 using System.Threading.Tasks;
 using Shouldly;
 using Volo.Abp.Application.Dtos;
+using Volo.Abp.Validation;
 using Xunit;
 
 namespace Acme.BookStore.Books