The 4-40 bolt is .108 aprox. 2.7mm
The 6-32 bolt is .135 aprox 3.4mm
I see you found the thread in the "Classic Pattern" area. Here is a shortcut
http://www.rcuniverse.com/forum/m_3735205/tm.htm
It occourred to me that these guys grease the majority of their landings. I would choose bolt size accordingly.
Greg