its is totally personal
I have had one put back 3 times and two put back once
The only time it worked was when i had two put back
However . . . tommorow i am going in to have 1 put back again. after being through a twin pg and the stresses of prems- it wouldn't be fair on my twins to have twins again deliberately.
twins are alot more stressful- not only after they are born but the risks associated with complications etc is alot higher
it is really up to you if you are prepared to take that risk or not - esp when you have other kids to consider.
don't get me wrong - i love having twins - but pg and first year were really hairy!!!
also going from two kids to four is a big financial step - new car - house etc

good luck with whatever you decide!!!
HTH and take care