OKay, I fixed that problem. The dll was 4.2.0 but in the web config, it was referred to 4.2.0.1. Fixed that.
Now it's giving me: Could not load file or assembly 'YAF.Core, Version=3.1.3.0, Culture=neutral, PublicKeyToken=8b49dc47f4701f66' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040). The version of the YAF.Core.dll is 3.1.4.0. Where can I check to see if that is correct?