Most ship types aren't exactly fixed size, most are categorized by sail rigging. Build a larger ship, have more sail and storage, need more crew. But it's still the same type of ship. So for example saying a sloop will need 4 people and cost 3000 gp will be a very stereotypical simplification. There will be smaller and larger sloops.
A lot of ships can be used with a crew of 4-6. Most Sloops, Caravels, Cutters and Feluccas would match the setting and the amount of sailors. Note that the characters should have some experience sailing.
Pathfinder as well as the D&Ds were always somewhat short of rules on boats and ships. For pathfinder there is a book called Admiral of the high seas that has a lot of ships, descriptions and prices. I haven't used it for Pathfinder as it contains rules for both Pathfinder and D&D, but it works well for D&D, so I guess it should be good for Pathfinder, too.
For example, a cutter, with a full crew of 4 and a minimum crew of 1 costs 1000gp in said book. To man this perfectly at all times, you'd need 10 people, so a crew of 6 seems reasonable if you don't expect fierce sea-battles or raging storms.