You didn't say
WHICH RZR 4 900 you would have. Apparently there are several models with different dimensions. Some are 138.1" x 60" x 71.7" x 1476 lbs. and some are 146" x 64" x 75.75" x 1565 lbs. That's 11'-6" long x 5' wide x 6' tall and 12'-2" long x 5'-4" wide x 6'-4" tall.
What is the payload on your truck? I have a '13 F150 with a 1607 lb. payload. I have weighed my rig twice loaded with bikes. Once with two dirt bikes (about 500 lbs.) and no water it had 780 lbs. on the tongue. The truck rear axle was 50 lbs. under the rear axle weight limit. The truck was at 99% of the rear GAWR and 97% of the GVWR. With two street bikes (850 lbs.) and 36 gallons of water (300 lbs.) it had 820 lbs. tongue weight. The truck was at 102% of the rear GAWR and 97% of GVWR. The rear axle was 70 lbs. (2%) OVER the weight limit. It needs a WDH head tilt adjustment to get some more weight off the rear axle, but I couldn't do that at the truck stop.
Without a HD payload package on the truck, a toy hauler big enough for a four seater RZR is going to eat up the rear axle weight capacity of a half ton truck quickly, especially with 5 people in the truck.
I have a 24.5' toy hauler that has a yellow weight sticker that shows 5025 lbs. The '"brochure weight" was 4650 lbs. I have not weighed it "empty". The two times I weighed it, it produced 780 lbs. (11.6%) and 820 lbs. (11.4%) tongue weight.
It only has 11'-2" from the back door to the kitchen cabinets, so it couldn't haul even the smallest RZR four seater. Also it only has 5'-6" clearance below the rear lift bed, so it wouldn't fit under there either. My camper has a flat roof. I suspect you would have to get one with a raised roof in the back to fit the four seater under the rear lift bed.
I hope you can find one big enough for a four seater that doesn't overload your truck.
It took me 53 weeks to find this one. I wanted to be able to haul three dirt bikes and have a bed available without unloading and not overload my truck. This one does all that, but just barely within the weight limits of the truck. The Ecoboost has no trouble pulling it, but the suspension is LOADED. Let us know if you find one that fits.

