source code for Microsoft enterprise library for version 4.0

Hi vicky-3979,
Since Enterprise Library 4.0 was released in May 2008, and it is already a retired version. Therefore, the documentation on Enterprise Library 4.0 is not updated and maintained.
I found some documents about Enterprise Library 4.0 you can refer to.
Enterprise Library 4.0 for Visual Studio 2008 released!
Microsoft Enterprise Library 4.0 – May 2008
>>upgrade the .Net framework to 4.8
First, you need to be sure that have installed the .NET Framework 4.8 , and then the next step you need to do is to change your project's target framework to 4.8.
Right click on your project in Solution Explorer, click Properties and in the Application view, change the Target framework to .Net Framework 4.8 by the dropdownlist below.
More details you can refer to this document.
Best Regards,
Daniel Zhang

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As the NuGet page of the package does not have link to GitHub, and given this library was released at 2008, it's unlikely to have the source code made public for this project.

There is one chance though, although I'm not very recommend it... use "ildasm.exe" to disassemble the 7 DLLs in the package

md Microsoft.Practices.EnterpriseLibrary.Common
md Microsoft.Practices.EnterpriseLibrary.Data
md Microsoft.Practices.EnterpriseLibrary.ExceptionHandling
md Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging
md Microsoft.Practices.EnterpriseLibrary.Logging.Database
md Microsoft.Practices.EnterpriseLibrary.Logging
md Microsoft.Practices.ObjectBuilder2
ildasm /tok /byt Microsoft.Practices.EnterpriseLibrary.Common.dll /out=Microsoft.Practices.EnterpriseLibrary.Common\Microsoft.Practices.EnterpriseLibrary.Common.il
ildasm /tok /byt Microsoft.Practices.EnterpriseLibrary.Data.dll /out=Microsoft.Practices.EnterpriseLibrary.Data\Microsoft.Practices.EnterpriseLibrary.Data.il
ildasm /tok /byt Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.dll /out=Microsoft.Practices.EnterpriseLibrary.ExceptionHandling\Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.il
ildasm /tok /byt Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging.dll /out=Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging\Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging.il
ildasm /tok /byt Microsoft.Practices.EnterpriseLibrary.Logging.Database.dll /out=Microsoft.Practices.EnterpriseLibrary.Logging.Database\Microsoft.Practices.EnterpriseLibrary.Logging.Database.il
ildasm /tok /byt Microsoft.Practices.EnterpriseLibrary.Logging.dll /out=Microsoft.Practices.EnterpriseLibrary.Logging\Microsoft.Practices.EnterpriseLibrary.Logging.il
ildasm /tok /byt Microsoft.Practices.ObjectBuilder2.dll /out=Microsoft.Practices.ObjectBuilder2\Microsoft.Practices.ObjectBuilder2.il

And then do global replace with VSCode on the folder on

" .publickeytoken = (B7 7A 5C 56 19 34 E0 89 ) // .z\V.4..
.ver 2:0:0:0" to
" .publickeytoken = (B7 7A 5C 56 19 34 E0 89 ) // .z\V.4..
.ver 4:0:0:0"

also

" .publickeytoken = (B0 3F 5F 7F 11 D5 0A 3A ) // .?....:
.ver 2:0:0:0" to
" .publickeytoken = (B0 3F 5F 7F 11 D5 0A 3A ) // .?....:
.ver 4:0:0:0"

and use "ilasm.exe" in .NET SDK to compile them again.

ilasm /dll Microsoft.Practices.EnterpriseLibrary.Common\Microsoft.Practices.EnterpriseLibrary.Common.il /res=Microsoft.Practices.EnterpriseLibrary.Common\Microsoft.Practices.EnterpriseLibrary.Common.res /output=Microsoft.Practices.EnterpriseLibrary.Common.dll
ilasm /dll Microsoft.Practices.EnterpriseLibrary.Data\Microsoft.Practices.EnterpriseLibrary.Data.il /res=Microsoft.Practices.EnterpriseLibrary.Data\Microsoft.Practices.EnterpriseLibrary.Data.res /output=Microsoft.Practices.EnterpriseLibrary.Data.dll
ilasm /dll Microsoft.Practices.EnterpriseLibrary.ExceptionHandling\Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.il /res=Microsoft.Practices.EnterpriseLibrary.ExceptionHandling\Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.res /output=Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.dll
ilasm /dll Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging\Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging.il /res=Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging\Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging.res /output=Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging.dll
ilasm /dll Microsoft.Practices.EnterpriseLibrary.Logging.Database\Microsoft.Practices.EnterpriseLibrary.Logging.Database.il /res=Microsoft.Practices.EnterpriseLibrary.Logging.Database\Microsoft.Practices.EnterpriseLibrary.Logging.Database.res /output=Microsoft.Practices.EnterpriseLibrary.Logging.Database.dll
ilasm /dll Microsoft.Practices.EnterpriseLibrary.Logging\Microsoft.Practices.EnterpriseLibrary.Logging.il /res=Microsoft.Practices.EnterpriseLibrary.Logging\Microsoft.Practices.EnterpriseLibrary.Logging.res /output=Microsoft.Practices.EnterpriseLibrary.Logging.dll
ilasm /dll Microsoft.Practices.ObjectBuilder2\Microsoft.Practices.ObjectBuilder2.il /res=Microsoft.Practices.ObjectBuilder2\Microsoft.Practices.ObjectBuilder2.res /output=Microsoft.Practices.ObjectBuilder2.dll

Now you can go on to include them in your .NET 4.X projects and see if anything breaks. (Although if there is no error in compilation, I believe there is high chance that it would work.)

Note: I've tested the recompilation works, however I have no project on hand that uses these library, so I'll leave it to you to test if it works.