.net 4 dll works... but only sometimes. When can/cant i use them?

I need to use 3rd party .net 4.0 dll’s from an Autocad package. I put in a Geometry library as a test. This worked well so i tested an in-house dll that referenced the Autocad dll but this caused it to fail.

Before i commit to porting my project, i want to fully understand what does and does not work so i know if it is even possible with Unity.

My guess would have been that 4.0 dll’s that don’t use 4’s features will work but my dll failed for including another dll that normally does work on its own.
So what is the rules for using 4.0 dll’s? Does referencing them (with minimum use) in a test project and not having errors suggest that it will work for my whole project?

Still cannot answer this perfectly but as a general rule i never use .Net4. I am not sure why it sometimes seemed to work but it may have been late error messages. Sometimes dropping a dll int the project told me there were problems immediately but often the error was only triggered after creating a new script or referencing the added dll.