Scott would be the better one to answer this as he had a 4000.
My (partly) uninformed opinion:
The HT3550 has a big resolution advantage but, despite how much better the BenQ is than it’s predecessors, I don’t expect it to keep up with the Epson’s blacks/contrast.
Everything else is tit for tat:
The Epson has lower lag but lacks the HT3550’s full bandwidth HDMI and crisp motion handling.
The HT3550 will auto detect HDR (instead of forcing you into the menu) and it’s MEMC works in 4K. Meanwhile the Epson has a motorized lens with memory that makes it possible to run a 2.35 screen without a costly A lens.
The HT3550 is small and handsome and makes too much noise in it’s normal lamp mode. The Epson is large and heavy and ugly and is way more placement flexible.
Both have DCI, both do HDR. The Epson has been lauded for it’s DCI-P3 color, meanwhile BenQ is getting plenty of early praise for it’s HDR PRO tone mapping.
They’re an interesting pair. I’m not convinced these two are competitors considering how much space separates them in price but I could be wrong. It seems like Epson is positioning the 4010 as a poor mans JVC (that’s a compliment) as I don’t see the 4010 outside of a dedicated home theater (can you imagine the Epson sitting on someone’s coffee table?). Meanwhile, BenQ is trying to assert themselves as the performance/value leader in 4K the same way they have in 1080p for so long. The HT3550 hits a price/performance sweet spot that it should appeal to enthusiasts and casual users alike.