Eekers! We're going from 1 to 3 rather than 2 to 4 but I had many similar worries relating to comparing baby twins to how DD was.
You may not be able to have a water birth due to needing to be monitored, and a C section is more likely, but around 40% of twins are born naturally so it's by no means a given. If your other 2 came naturally there's a really good chance the twins will too. I BF DD and again was a bit fixated on this but I'm on some useful FB groups and can see that plenty of people manage to BF twins.
We had a bednest for DD, we are using a cot this time with the side off and attaching it to the bed and putting both in there to start with. DH did offer to have a bednest on his side too! but everything I've read says they are usually fine to sleep together at the start and some stay in the same cot / bed for a long time, and it can help them sleep better than singletons (haha we can only hope!)
As for affording them... depends on your situation. We actually can't really afford childcare for them the same way we did with DD (in nursery 2 full days from 11m), and I'm probably going to be giving up work. But in terms of actual stuff for them it probably depends on all your kids' sexes and how much you care about things being new! We've got virtually everything secondhand, even an extra car seat from a trusted friend. Ours are both boys, and friends are offering us bags of clothes for them, I don't think we'll have to buy much.
Lets be honest it is going to be different! but I think when you've done it once or twice before you do have an advantage once the shock has worn off, as I think you have already got the first time mum stuff out of your system and also aren't so focused on proving you're a good mum, I am personally biting off every hand that offers help before they can even say "I could do...". My mum's coming to stay for a couple of weeks (which I would've hated with DD!) and a couple of aunts have offered too, friends have offered to take DD on days out etc.
How old are your others? I was just glad that we didn't need a triple buggy!! (DD just turned 3 and is finally walking reasonable distances...)