Howdy, Welcome aboard.
You'll need to provide a little more info........Like the model number.
OMC offered 2 types of (cone-clutch and dog-clutch) drives in from 1993 to 1994.
You could have a 93 model because Four Winns (and most manufacturers) frequently use "last Years model" engines and drives in their boats.
Have a look at
http://epc.brp.com to find your sterndrive year and model number. You have either a 93 or 94 model.
Your immediate problem could be a slipping prop hub like suggested above or a failing coupler. If it's the latter, it won't slip for long. It will fail completely very soon.
Otherwise it might be a internal problem in the drive.
It could also be a shift cable adjustment problem but that won't last long either. popping out of gear usually damages (permanently) the shift dogs......it's especially damaging if it does it at high power settings (like anywhere above idle) If that's what it happening, DO NOT let it do it for very long or you WILL be rebuilding/replacing the drive!!
Regards,
Rick