Our four are a bit more spread out than others.
So some of it is a bit different, for example by the time DD2 is doing sport, DD1 will probably be gone from home, or certainly much more independant.

But FWIW I agree that a 4th doesn't impact much more than 3...I found the step from 1 to 2 to be the biggest, each additional child has been less of an impact on us financially.

The biggest thing was we needed a new car, but we really needed one anyway... we got a 4yo Kia Carnival & it's pretty good...we like it.

I think a lot of the costs are gradual, they just slip in IYKWIM? we don't really think about 'well if we didn't have to buy this or that then we could...' because it all gets eaten up somewhere in the end.

Sport.... DD1 dances, so it doesn't interfere with DS1's football. DS2 wants to do swimming... but TBH we find that football on a Saturday takes up a huge chunk of the day anyway. The hardest part would be if the games are scheduled at different times & different fields... but you could be facing that with 3 boys anyway