UPS Requirements - Max load or actual load?

Measure the real utilization of your systems infrastructure at present and build in room for expansion, spikes in utilization and normal growth.

There's nothing more than that.

As for UPS design, it depends. Dual power supplies are good to have, but what do you want to protect against? How did your actual UPS fail?

System power ratings are misleadingly "inflated" to account for maximum potential system load. Ie, if you fill all the supported bays and banks and slots and PSU positions, and then everything runs at max load.

In practice, actual load is lower. Usually it is much lower.

So you should measure actual power draw for your running gear. And then add a margin for the spike that occurs when the hardware is initialized at power-up (if you didn't have a chance to measure this in reality). When I can't measure the whole room/rack/facility booting at once, I semi-arbitrarily go with 30% over the running power consumption. Then round up to the nearest UPS size.

Et voila.

Servers can require significantly more power during start-up. I've seen a data center blow fuses following a power failure when the normal draw was about 30% of the fuse rating. We ended up staggering startup delays on the UPes.

At 4000W it is likely you are on a three-phase feed. You will need to do some planing and testing to ensure the load is balanced.