My DS is exactly same age as yours. We dropped the nap 6 months ago and some days he is so, so tired - especially at weekends. But ANY sleep after 2.30pm is a disaster for us - even a few minutes over, and he will be awake until 9pm or 10pm and still up at 6.30am next day but then grotty and tired. We can still manage a short nap at the weekend. I also find DS nods off in the car and it is impossible to keep him awake if he is exhausted so as far as possible I'd avoid longer car journeys while you are training new habits!
At this age, ive read that a 6am wake up is very common. But 4.30am is not sustainable for anyone. No wonder you are exhausted.
It was hard but we planned some days where you don't let DS sleep in the day - in the afternoon good gentle activities, outside in sunshine the afternoon helped a lot - eg chalks on the driveway, play with a big tub of water and some plastic pots and pans, teddies picnic at the park, take your regular toy cars or PlayDoh outside in the garden and play on a rug.
I find that there is an optimal amount of stimulation and exercise in a day - too much and my son wants to be in bed by 7pm which leads to a disrupted night and a very early start. Too little and I have to go on a campaign to wear him out before tea. Often we end up going for a bike ride to the park between teatime and bedtime which doesnt help as he gets over stimulated and resists bed, I'm still working on that!
When ds is tired we have tea finished by 6.00pm, get him in a long bath and drag out stories,milk, cleaning teeth, we listen to music he likes to settle him for bedtime 7.30pm. It works generally but often he is still awake at 8.30pm, sometimes 9pm. And he will get up once in the night still, majority of days. But he now sleeps til 7am most days. Yayyy.
Whatever pattern you choose you can and you WILL change his sleep pattern. But that nap HAS to go, aiming for at least 5 days a week with no nap at all. Im sure that's the key to it.
Worst case scenario: Go to bed early yourself and expect him to be up early but if/when he wakes up before 6.30am if he won't settle then you can bring him into your bed, don't engage with him, and let him play with a teddy or fall back to sleep.