Thank you 💐.
It really does depend so much on your DC too. OP, my high needs child came second (and at a difficult time) so it also feels harder because my first was a dream baby comparatively speaking.
If your first has been high needs, it may well be that like many other posters on here your second feels much easier in comparison. It's not impossible but you would be very unlucky to have the same experience again.
I stress though, set help up before you need it, not after you find yourself not coping. Get a cleaner if you can and make sure you have someone to watch the kids for a few hours a week, a sitter, neighbour, family member etc. If there's no one obvious to ask, take time now to familiarise yourself with local child minding sites etc where you could potentially find someone looking for a few hours work here and there or on a regular basis.
At 4yo, your DC1 will be much more self sufficient, I'm sure able to watch tv or maybe play independently. He will also be at school soon of course which will make an enormous difference. In the holidays, could you also arrange play dates for him where you drop him off at friends houses and have a little space?
I understand you're apprehensive after your first experience, but like I said, if your main focus over the next year is the family and you don't have any other major commitments, with that age gap it's perfectly possible to have a really great time and enjoy life together.