RE: I want to build a boat
Prop Size varies too much to be specific. That's why I constantly say to ask if unsure. It's often the final decision for a power system and can often make or break the setup. Choose the wrong one and your new system will be nothing but smoke.
To be honest, 4s in a 25" hull is a lot of power. If you wish to go this route, utilizing the system that comes with the Aquacraft SuperVee 27 on a 40mm prop would work well. This would be as much power as I'd use on a 25" mono.
Ryan