4 of 7 of mine climb on occasion.

"Starting in the early evening, even if the lights are still on, ball pythons will emerge from their shelters. If hungry they will hunt and may even climb on branches close to the ground." Philippe de Vosjoli 'The Art of Keeping Snakes'

"Despite being heavy bodied, Ball Pythons can climb adeptly. We there-fore provide a minimum cage height of two feet and elevated perches." Bartlett and Bartlett 'Snakes'

I'd also say that 4 feet is likely too high I'd suggest that 16-18 inches is quite enough. I have no problems with Humidity or heat at all in a 30x24x18 enclosure. It is all down to how it is set up. Well sealed plywood should not be too much of a problem but well sealed is the key. I'd usually recommend paint with a drying time of at least a week and personally I'd go with two and as warm a temp as you can manage cool temps slow the out gassing of paint. If you have deep pockets a liner polyurethane (2 part) paint cannot be beaten but it is very costly. Have you looked at using PVCx ? it is an extruded plastic works like wood and can be joined similarly it isn't super expensive either. Slightly more than ply.