Well, actually, when you jack up the car you will be supporting more than 1/4 of the weight of the vehicle. More weight is on the front end of the vehicle. You will find that even the rear wheels will start to lift off the pavement to some extent when you lift from the front of the vehicle toward the rear of the jack point. If you lift only from the rear lift point, it would be true that 1/4 or even less of the weight of the vehicle would be on the floor jack. The typical weight distribution is probably about 62/38 front to rear on most front wheel drive vehicles which is 98% of passenger vehicles today.
The important thing to bear in mind is never to get under a car using only floor jacks. I don't even truly trust jack stands unless on perfectly flat surfaces as the weight can shift and disaster results. By using the metal ramps the car isn't going anywhere which is why I use them to change the oil. Tire rotations are no problem because I never get under the car. And that is why I use two of them so I don't have to keep jacking the car up and down using only one jack. The heavy duty garage quality floor jacks are simply too expensive for a home owner to invest in. Of course you can always find the cheap Chinese made jacks but I don't trust their quality for tools. But really the best deal, at least for me, is to take it to the local Hyundai dealer on Wednesdays where I can get a complete oil and filter change for $19.95 and he disposes of the oil and filter. It costs me that much just for the supplies at the local auto parts store. But I prefer to rotate my own tires and check the air pressure and torque the wheel nuts down using a good torque wrench. I don't like those air impact wrenches and even with a very long breaker bar it is very difficult in some cases to break the nuts loose and occasionally will find one that is only finger tight.
