We felt the same, wasn’t 100% he’d be safe in the sat up seats so we hired a Doona for this first trip (also helped with the issue of car seats!) and will look at other holiday options when he’s a bit more robust! X
Suggestion above is also a good option but just remember you will be restricted to leaving baby in there for up to 2 hours only X
4 months and that was an absolute stretch 😂 she hated it since 2 months old. 4 months is fine, just recline it. The leg area usually lifts up as well so their legs aren’t dangling. X