Wow, there's a question!
So, you have 3 and you're thinking of 4 but it could turn out to be 5? What ages are your existing children? That's quite important in the consideration IMHO.
I'll link this in to the multiples daily thread for more responses.
Personally...........I conceived my DTs naturally and felt extremely lucky throughout my pregnancy and for the first 6 months. Then I kind of went downhill as I had done far, far too much (moved house twice, partner worked away, tried to everything without asking for help etc etc) and I spent some time wishing I'd never had two babies at the same time.
Now? My DTs are 4. Yes, its been hard work and very, very tiring (we have an older girl of 7) but I am so glad to have had them, so lucky to have had the experience of twins. I feel most at home with the girls on here who know what I mean.
The joy of hearing my twins talk about "us" and "its going to be our birthday soon" fills my heart. When one is hurt and the other brings his/her teddy and passes on a hug or when, after bedtime stories one comes running back through to say "oh, I forgot to hug DT2", the list goes on. Its fab!!!
Don't get me wrong, my DTs argue like cat and dog or any other brother and sister. But they are also a team and its gorgeous.
My vote (and its only one opinion) is to take a deep breath, go for it and see what happens.
To look at it another way: Would you always wonder what it might have been like to put in two if you only put in one? Or would you have total peace with one more child?
Keep us posted! [nosey moo}