How do you check historical availability?
I see no problems with Availability in AF/KL website, but it is not cheap... 180K miles + ~400€ for one way, also some for 180K miles + ~200€ or 100K miles + ~400€ when looking for one ticket instead of 4, so it might be smarter to book separately.
This is for next June BTW, since you can't book anything more than 10 months in advance with FB miles.