For some reason I just can’t find it in the rules. Aren’t you supposed to only use like 3 shipping lanes with 3 players? 4 with 4 players etc ?
No. Full board always.
My experience is that that is not just fine with 4, but that 4 is a better player count for the game using the full board than 5 is.
A reduced board for 3 would make some sense but it is not part of the rules, would be complex to tack on, and is fundamentally not necessary.