There is an 18mth age gap between 3 and 4 ( only 6 mins between 1 and 2 LOL)
I use and still do a side by side twin pusher, did use a sling when #4 was little

As I had twins first I already had two cots, #3 has just gone into a todler bed now ( at 3)

We had two car seats which they both still sit in to this day.

rest when bubs rests for coping being pg whilst you have a little one.

Dont feel guilty about the choice of having them close together, if this is what you want go for it, you will cope no matter how old they are.