Alexa is right. I have done more M&B + take off/landing distance calculations *after* my skills test than before. And I have less than 100 hours total, including the PPL course...
In general, unless you go for something like a Cessna 182 or Piper Archer, most four-seaters are indeed 3-seaters. Ask your relatives to shed some weight while they can, ask them to pack light (10 kg baggage limit all round) and only then expect to be able to legally take off with full fuel.
The following data is for a typical PA-28-161 Warrior:
Empty weight 1550 lbs, MTOW 2440 lbs. Full fuel is 48 USG/289 lbs. (Fuel to the markers is 34 USG/205 lbs.) So with full tanks you have about 600 lbs payload to play with. Three average male adult passengers is 549 lbs (according to the FAA), leaving 51 pounds (a little over 20 kg) for baggage. And do you believe the average male passenger is only 183 lbs/83 kg?
Typical 65% cruise consumption (about 100 kts IAS) of the PA-28-161 is between 8.8 (not leaned) and 7.5 (correctly leaned using the WOT method), so 48 USG gives you an endurance (including reserves) of a little over five hours. If you only fill to the markers (34 USG), you have a tad under four hours endurance, but about 85 additional lbs payload to play with.
Balance, I have found, is only a problem with 2 heavies in the front and nothing in the back. As soon as you have a back seat passenger, balance is fine. And I have never seen a situation (so far) where I have had a CoG that was too far aft. Then again, I have not flown each and every type you'll find on the average rental market, in each and every possible configuration, so check anyway.
As far as landing and take-off distance is concerned, remember that the POH figures assume a new, clean aircraft with a new engine and prop and a highly experienced test pilot with perfect flying technique. Neither of these five factors will be true in your situation so take them with a big grain of salt. Don't fly into a short strip, hot & high, at the end of a long leg, for instance. And if you know that the landing distance is going to be challenging, don't hesitate to throw away the approach (go around) if it doesn't look perfect.