I think it's a big ask on two counts, firstly the bride and groom marrying abroad and expecting their guests to make child care arrangements for baby/child/children for what might be as long as four days. I think if you marry abroad, you must expect that some guests will decline the invitation, it's quite a big ask really just from the expense side. It depends on how old and robust the grandparents are, a few days of childcare for a baby/toddler can be incredibly tiring. We have ours overnight fairly frequently now they are children, maybe the odd night when they were babies, certainly not 4 nights on the trot. A very big ask and your anxiety, not unreasonably, is palpable from your post.
It's a minefield really, grandparents are expected these days to play a large part in child care, some are really happy to do that but some feel put upon, you are doing quite a lot already OP. I agree with notanan about the parents "pushing their luck and appreciating what you do already" and as quizqueen has succinctly pointed out when you are a parent "you can't just do want you want anymore" sacrifices do have to be made. I remember having to cancel a birthday week-end away when one of our children fell off a climbing frame at school, we were told on his head, and having taken him to the doctor her advice was, it would be advisable to be on hand over the next couple of days, as it happened he was perfectly fine. That's life though! goes with the territory of being a parent I'd say.