Since you have added Service4 class library reference only in .NET 4.0 dependencies section (refer below snapshot), You need to use Conditional Compilation.
To access, Service4 members, you need to do like this-
#if NET40
Service4.Test.PrintHi();
#endif
These are list of preprocessor symbols used in #if directives:
.NET Framework 2.0 --> NET20
.NET Framework 3.5 --> NET35
.NET Framework 4.0 --> NET40
.NET Framework 4.5 --> NET45
.NET Framework 4.5.1 --> NET451
.NET Framework 4.5.2 --> NET452
.NET Framework 4.6 --> NET46
.NET Framework 4.6.1 --> NET461
.NET Framework 4.6.2 --> NET462
.NET Standard 1.0 --> NETSTANDARD1_0
.NET Standard 1.1 --> NETSTANDARD1_1
.NET Standard 1.2 --> NETSTANDARD1_2
.NET Standard 1.3 --> NETSTANDARD1_3
.NET Standard 1.4 --> NETSTANDARD1_4
.NET Standard 1.5 --> NETSTANDARD1_5
.NET Standard 1.6 --> NETSTANDARD1_6
For more details, refer this article
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