Being 16 I assume you want a fast 2 seater? You can get a nice Yamaha GP1200r or GP1300r for $3500. As well as a 2000-2003 SeaDoo RX.
You can also try looking for a 4 stroke 2004 SeaDoo RXP but it will be tough to find one for under $4000 that's not beat up.
State what your priorities are: Power, stability, fun to ride, 3 seat capacity? etc...