Hello Microsoft...
Like many other customers, I was stung by the April 19th firmware update. I use(d) a Surface Pro 4 connected to a Seiki SE39UY04 39" 4K monitor via miniport, through a Surface dock.



I used the SP4 as a primary desktop computer, and moved it 2-3 times a week to our conference room and used an HD monitor that was only 1080P capable.
The SP4 had no problem scaling resolution between the different external monitors, up until 4/20/16, when my SP4 installed the software and firmware update. After that, I could only get 1080P on my Seiki monitor, which is an unusable resolution.
I tried another Seiki monitor in our office and the SP4 could not scale the resolution which made it seem like the problem was with the SP4. Another coworker plugged an apple laptop into the Seiki and it immediately scaled to 3840x2160, so the problem again
seemed like the SP4.
Not having any guidance from Microsoft (and since the firmware was burned in and couldn't be reversed), I decided to painstakingly replace my heretofore perfectly good SP4 with a Surface Book. Thinking the software updates were part and parcel of the problems
I was having, I did not do a backup-restore, instead starting with a "clean" SB. This required several hours of software installation and password transfer etc. A crappy option considering I'm not an IT guy.
So, thinking my problems were solved, I took my SB to the office this week and I got more of the same. No scaling to 4K, stuck at 1080P.
The Microsoft store was partially sympathetic and gave me a full refund for my SP4 and I spent around $500 to upgrade to a Surface Book. Unfortunately this didn't solve my problem.
Today the Microsoft Store (White Plains NY) called and asked if my problems were solved, I said no. They asked me to come in and they would help. Before I left my office, I plugged the SB into a Samsung 27" 4K monitor and it scaled to 3840x2160 immediately.
Having checked everything possible, (wires, dock, monitor, and computer) it now seems the only possible cause is the Seiki 39" is not longer supported by the Microsoft software/firmware update of 4/19. Basically, a Microsoft initiated fix rendered my monitor
incompatible.
I asked to Microsoft store to replace my separately purchased monitor and they laughed. I now have the choice of purchasing another monitor out of my pocket....or pounding salt. Words cannot express my despair.