I'm trying to convert quite big project from .net 3.5 to .net 4.0 and we have wwf 3.5 in it. Googling this question i realized that i would need to rewrite workflow almost from scratch. so my question is , if i can use .net 3.5 workflow in .net 4.0 project ?
Yes, you can. WF4 doesn't replace WF3. They are in different namespaces. The blog post isn't on topic, but the blogger is and he states this fact.