Well, I can't comment on the reality of 4 young kids as I have much bigger age gaps.

But as to the practicalities...ATM we are in a 3brm townhouse...it is squeezy but we manage.
And I totally agree with Trillian...they cost as much or as little as you want. we always manage to find enough. One sport / activity per child is enough for us - but that depends on the family really.

WRT cars... if you had a second car that wasn't getting used much... it wouldn't matter if it wasn't a 7 seater, would it? Our second car is a 5 seater (very old Laser) DH uses it for work, if we go out as a family it's in the Kia. We don't need two 7 seater cars cause we all fit in one IYKWIM.

Mind you - all of this is coming from someone who is trying to convince her DH NOT to get the snip just in case we might have more... he says it's not practical for us & I say well we would have stopped at two if that's what it was about

Wishing you LOTS of luck with your decision