What size is that small screw that is on the pull handle on the inside of the cab, that screws in from the bottom?

It screws in right in behind where the threaded part of the cable is at.

Pretty sure m4, the length may be 4 but not positive

Does the M4 stand for metric 4mm?

Not sure if the one that was in there was correct length, but appears it is used as a stop?

Anyway, for future reference, after removing the speaker, it's a good idea to stuff a rag in that hole behind the speaker, so if the screw drops, it don't go down into that black hole.

It’s always good to have a magnet on a stick for those sorts of issues.