I'm like you - we want another one and would like to have him/her as soon as possible because my 3 girls are so close in age, I don't want 1 little one left as the odd one out who's too little to play with the big girls.
DH is terrified at the thought of having twins again also (have you ever tried to find a car with FIVE anchor points?!!). Although, as ours are identical, the chances of having more twins is very remote.
Good luck with your TTC and hope your twins get a new sibling sooner rather than later.