The first cage is good for 2-3, the second cage is good for 3-4. Make sure to give them enough perches and a extra food bowl if you do get more. You might want to keep the current cage as the new birds may not get along with the birds you have now. Its also a good idea to quarantine the new birds for two months, let them get to hear each other and then get to know each other slowly.

No, keep only 2 or they would feel cramped. That's not a very large cage, so just keep it to 2 so they can be happy.

Also I love this one

It's more expensive, but would it be big enough for four budgies?

They aren't socialized so they'd be spending all their time in there.

http://m.petsmart.com/h5/hub?id=bird%2Fc...

EDIT

I FOUND AN EVEN BIGGER, cheaper one!

http://www.amazon.com/gp/aw/d/B00176F5L0...

And it has great reviews!

Size 26"L x 14"W x 36"H

I'm upgrading my parakeets' cage, and though I know it's big enough for my two, I feel like they'd be less lonely with more friends?

But I've heard that 3 budgies is a bad idea, and I'm not sure it's big enough for 4.

Any advice?