Motion Plus - an adapter about 1 inch long that plus into the Wiimote - retails around $17.99-$19.99
You can get a Wiimote that comes with a Motion Plus as a combo for cheaper than buying each separately, so if you need a 4th Wiimote this would be the way to go.
You'd also need an extra nunchuck to go with the 4th person.
So if you buy Wii Sports Resort with one Motion Plus (lot of places have it on sale for $39.99 right now), you would need to buy:
1 extra Wiimote/Motion combo
1 extra Nunchuck
2 extra Motion Plus
Wii Sports Resort would provide the 4th Motion Plus
Sports Resort is quite fun - big improvement over the original Wii Sports if for no reason than having like 12 games instead of 4 games, plus the add on extra levels you unlick (just like the original).
Alas, the Motion Plus is currently really only used in about 4 games. Hopefully they'll make more in the future. For games that use it you must have it to play them, for other games the MP does no good at all since the game programming doesn't use it so you may as well take it off for those games (though you can certainly leave it on).