on leash walk? or off leash run? Honestly, a walk is just warm-up for an active breed dog, just a chance to work out the kinks before the REAL exercise starts. At your pup's age, my boy was doing at least 45 mins of running twice a day, then his walk of 3/4 mile, then short obedience several times a day. And that was on the weekdays! Weekends is when we really pulled out the stops - 3-4 hour off-leash hikes or trips to town for him to take in all the new sights and smells. Now, that much exercise is barely enough to keep him contained. Then he has obedience class. And conformation class. And "meet the public" demos with our kennel club. etc etc
Does he like to play fetch? Does he like balls? Get 2 throwing toys or balls. Chuck-its are awesome for this. Throw the first one. When he brings it back, immediately throw the second. You don't want to over-do the running with a young pup, but you can get him tired pretty quick. Also, please don't forget the mental stimulation. Aussies are VERY intelligent as are all the herding breeds. If you don't give him a job, then he will find one. And his idea of fun is probably pretty different than yours. If Singe doesn't get his brain wore out, he likes to take all the shoes in the house and hide them in weird places - in cabinets, in the laundry, stacked on the basement shelves....
Teach him to pick up his toys. Work on "nothing in life is free". Play hide and seek with him and the family. You can even teach him to find people by name. Teach him the names of his toys and then have him go get it for you.
As I said, at 4 months old he isn't even hitting puberty yet. In a couple months, he is going to forget everything he knows and become a teenager. No coincidence that 6-10 months old is the age that most dogs go to the pound. You want to have a good solid foundation before then.
As for teaching come, don't EVER call him if you don't have the means to insure that he will come to you. Even one time lets him know "oh I don't have to unless I feel like it" Start with him like you would an 8week old puppy. Always on leash. Get a long line and keep it on him all the time. Have him wear a 6ft lead in the house (supervised of course!) that way you can grab him at all times. Get the whole family outside and work on "come" If he knows the basics of the command, then have everyone get in a circle about 15 feet apart. Tie a small weight on the long line so that you can toss it from person to person. Throw it randomly around the circle and whoever has it calls the dog. Use the line to reel him in if he doesn't come instantly. Get down on one knee and use your happy voice, encourage him the entire way. Most importantly, NEVER call a dog and then punish him. If you need to correct him, always go and get him.
Personally, most dogs do better in a class than one-on-one training. They provide distractions, a chance to socialize, and more different techniques to see. Another student might teach a command in a different way that would work for you. Plus, classes are a LOT cheaper.