Lots of slow hacking, up and down hills, natural ground, build up gradually and allow lots of time for things to sink in, give mini breaks of a few days every month at least for him to recharge the batteries and allow the new things to sink in. Keep school work to absolute minimum, it's easy to school while out hacking and better for the joints than continual turning on a surface. If there's a farm ride or something near you they're ideal as you can use their different terrain at different speeds to learn balance and agility while having a bit of fun and getting fitter in the process. It's harder for a youngster to get fit properly and use their muscles than an older horse so it needs to be done gently; little and often.
I do a three day rotation day one hack ridden or led no more than forty five minutes.
Day two Lunge or long rien perhaps with poles no more than twenty minutes Day 3 school work no more than thirty minutes then a rest day .
Then I gradually increase the time spent ridden .
I also use a basic fitness work like any other horse- start with 30-40minutes walking increase over 3-6 weeks to include hills for 2hrs at a time. then add in trot work for 3-5 weeks, same with canter. I back at the same time as I increase fitness tho!
I would be looking at a plan as less of a fitness plan and more of a gaining experience plan at this stage. With the correct regime, the horse will gain gradual fitness anyway.
Hacking is your friend here- some gentle sloping hills in walk, bit of road work. Keep school work minimal and low key at this stage, you can do just as much 'school work' out hacking.
Always keep in mind that a 4 year old is genuinely still growing and developing and you want to avoid anything that will put too much strain on the horse at this stage.
Most of all, enjoy building your ridden relationship up.
Week 1 -
Short hacks on the road to build up his leg muscles but gradually increase the trotting depending on how he's feeling. Start using the aids for contact on the way home. You'll find he'll want to pull and you using your leg and keeping a firm contact with your hands will help him to engage his hindquarters and round without him even knowing it. I would say 2 days off at this point, more depending on the length of your hacks. Twice a week take him onto the beach and let him let off some steam, make sure you use really clear aids so he doesn't get confused later on.
Week 2 -
Continue as normal but have one day with some light schooling, around 10 minutes but keep him busy. (I assume he knows the aids for walk halt trot canter etc? If not build up his skills with these.) Start riding down the center and 3/4 lines, teach him to balance himself and maybe add in a few 30cm circles in walk to begin teaching him to bend.
Week 3 - Build up slowly to 2 20 minute sessions a week and then continue for 2-3 weeks with as many breaks as you see fit for your horse (for instance if he's a work diva give him one but on the 2nd day do some very light hacking or if he really isn't that bothered give him 2 days off but spend one day doing some in hand work).
Week 5-6 :
Cut down less on the hacking and bring in some help, an instructor, or someone knowledgeable. They will be able to help you with exercises to help him soften, exercises to help him bend, understand newer aids for instance collection, contact, trot-halt. You can also begin to build up his shape knowledge as well.
Eventually if you carry on like this you will decrease the hacking, introduce some pole-work and begin to see a more rounded, soft horse. Remember to work to your horse, if he feels like he needs more than a week on the first step give him that or if he's catching on really quickly step it up a notch. Also remember that you'll have good and bad days, one day you'll feel like 'YES Break Through!' and others more 'Come on, you know this!' so give him a lighter workout after a more challenging one whether that be mental or physical. A instructor will be able to help you more with this though.
This worked with my pony and most is from an instructor I myself had and 3 years later I have the perfect horse!
But just remember to enjoy it - the thrill of riding an experienced horse hwo you know you made experienced is so amazing!
Good luck x