Re: whats the maximiun weight of motor can you put on the mariner 4?
If that boat is rated for a 3 HP engine, then go for the 3.5, depending on the brand, they weight the same or very close to each other. That boat being built with thin pvc and not counting with a transom will have stern bending issues no matter how well you got it inflated. So throttle with prudence to avoid excesive bending.
Happy Boating