The 4 Seasons is in the upper floors of the Mandalay, but is more family oriented.
It is a seperate hotel in the same building.
It has it's own reception area, restaurants and bars.
It has no casino.
It shares the same pools as the Mandalay, but the pools are totally family-oriented.
There's no 'Spring Break' kind of stuffs going on.
But it is of course connected with the Mandalay Bay and then by monorail to the Luxor and beyond, so if you want to go explore the Strip, that's totally possible.
We've used it on a few occasions with our family, and it would be my recommendation.