Does anyone have one of these 4 gallon portable vacuums? Curious how they perform

I am not. I am familiar with shop wet/dry vacs from my places of employment and the only one I brought for home use 15-20 years ago.
If you want a U.S. made workhorse with good customer support go look at Rigid brand vacs. My 6 gallon wet/dry has saved my basement from becoming an aquarium, keeps my drill press from looking like a scrap yard and stood in for my wife's big dollar house vacuum for a month until parts arrived. My experience doesn't qualify me as an expert, but if it ever dies I'll buy another.

what h.p. rating do they claim?

I dont have a dewalt, but i have a "5hp" 4 gallon ridgid. I dont use it for wet pickups, i use a full size for that. Anything else i use the smaller one. It is much more conveient to move around the house or out to the car.
I use a filter bag. There is almost no dust output even when vacuuming spackle dust.

I consider it money well spent, and would buy another when this one eventually dies. Mine is > 10 yo and running strong.

No. But my guys at work all love the Milwaukee ones we have.

I've had the 2 gallon Dewalt portable going on 2 years now. Works great. Although it can also plug in I think it actually works better on battery power. Definitely need the larger and higher amp rated batters if using for any length of time. 1 5ah 20v max battery will give two full car cleanings and leave you about 1/2 charge left over.