A couple of things. First I hope it works out for you. If the prop is not deep enough you’ll get cavitation or ventilation and then your rpm’s will increase and your thrust will decrease, meaning even less power.
I troll lots for different fish, trout, salmon, striper but not walleye as we don’t have them in California so I don’t know what speed you troll for them but I surmise it must be pretty slow if a 4 hp motor will push a boat your size at their trolling speed. Even if it pushes your boat fast enough the problem you’ll have is controll. The least little breeze will push your boat in the direction of the wind and I can tell you from experience you cannot correct your course with that little motor and your size boat. The only way to get back on track is to start the main motor to get you back on course. As long as the wind blows or even a little breeze you’ll have to do that over and over.
I have an 18’ Alumaweld heavy gauge aluminum boat with a 90 hp merc and a 6 hp merc kicker. My boat probably weighs 2500 to 3500lbs less than yours and has way less surface area for the wind to effect. When trolling in wind/breeze 6 to 10 mph I am constantly trying to keep the boat on course but find I often need to start the main motor to get the boat back on track. I wish I would have got a 9.9 hp. When calm and no breeze , no problem. Your boat being much heavier, more surface area and less hp it’s going to be a real hassle trolling (especially if your prop isn’t deep enough). But again I hope it works out just fine for you. Tight lines.
