It's not 100% clear that they exhausted other reasonable options. They don't say if the 4 employees were a flight crew that NEEDED to be somewhere. There's no indication if there were other employees (not in uniform) that could have been deplaned first. There are other flights, on Sundays, that leave after this one (one at 9pm).
The judgement to go with violence after offering $400, then $800 is odd as well. Surely someone would give up a seat after the next one or two bumps in the offer. The onus is on United, since they didn't identify the issue until after passengers were boarded.