Well , really he is a bit on the lazy side.
About your 4 yr old lab that is very unusual for this breed for they are naturally a working dog they are bred to work. I would deffinently get him to exsersize more make sure he is eating a wholesome natural diet ie no corn or by-products-if your having trouble with the sniffing thing on walks- like most dogs do specially if they arnt neutered i would recomend the halti or gentle leader which is like a halter for horse it gives you great control, has a calming effect and when he pulls his head will turn to the right or left-it discourages them to pull unlike the body harnesses that are available, get him used to water, go walk in a small creek on a nice day and bring him with you on a leash just stand there and let him get used to it and slowly go deeper and deeper labs love water-usually most dogs are wary of it cause they were never around it or had a bad experience. i rescue dogs and all of them are terrified of water from bad experiences but after a bit they get to where they are playing in it!!
good luck