A drift boat or river raft will handle four people quite easily but they are not ideal on a lake and most don't handle a motor well. A 16' John boat with a jet outboard will hold 4 and will work on a river or a lake, as long as the lake does not have large waves. A 16' Lund will work well on the Lake and decent on the river as long as you don't try to run too shallow. You can run them with a Jet Outboard or a standard prop drive if you are good at reading rivers and add a prop cage and guard, and run with the motor not locked in the down position.
Big trick on rivers is you want a boat light enough you can manhandle it is you need to.
More plush version