Shouldn't ... but has.
I called LH after failing to secure a seat online (I got an error, but don't remember what it was). LH agent took payment information. I didn't get the seat, though, nor was I actually charged. On check-in at the airport, I inquired on why I couldn't check-in online. I was told that's the case for all UA tickets on 4Y. This was a year ago - hopefully things have improved.
I have had no issues with 4Y on 016 tickets.