We have three year-old twins from a 2-blast fresh cycle and we wanted a sibling so went through the frozen embryos left from that cycle, having a chem pg (single blast FET) and a mc (2 blast FET). We did a fresh and again only transferred one (despite my RE reco to do two), as my twin pg was really, really scary (we almost lost them so many times) and I delivered at 34w. My DS has lots of lung stuff (every cold seems to last for months and turn to pneumonia or bronchitis) and my DD has severe reflux at 3yo still, that interferes with her ability to do various things (she loves gymnastics but chokes when she goes upside down). So even twins who arrive past the worst dangers can have issues that persist, which I really wouldn't wish on my worst enemy. I love my twins and if I could go back in time of course I would do the same thing again as I can't imagine life without either of them, but for one of the FETs and for the fresh attempt, we did only transfer one to give our next child the best chance at being healthy. I would not have been averse to raising another set of twins (though it would have been REALLY REALLY difficult even with DH a pretty involved dad), but I just didn't want to risk their health - and mine (I also developed preeclampsia and became very ill during that twin pg - it's much more common with twins). In your shoes especially, I would really, really, really only transfer one blast if your clinic has good stats and you will have some to freeze. Even with the best support system, if you have a twin pg and end up on bedrest (I was on bedrest the ENTIRE pg with twins), that's asking an awful lot of everyone else - including your four year-old who will miss so much time with Mom. My singleton pg this time (I'm now 35w+) has been sooo much easier and I'm healthy and present for my kids, which was the most important thing to me. Of course if you end up with twins you will probably all get through it and you will love them both dearly, but I really have to say... it's so much better when it's not just about "getting through it" but enjoying life in the process.
Anyway, I know that was very opinionated and I'm sorry if it comes across too strong, just sharing the benefit of my personal experience... since you asked for opinions
. GL to you, I hope you have a successful cycle and a beautiful, healthy baby! Best,