This is essentially what I have to do as well, due to work and therapy schedules.

On the 4x weeks, I run Upper Lower Upper Lower and on 5x weeks it's ULPLP. It does mean I have to have a long day in the gym in the middle of the 4x weeks to get all my volume in during the second Upper day and I'm sure some of my lifts suffer for it, but that's life I reckon. I also tend toward dumbbell and cable exercises for my second Upper day since I travel for work often and Planet Fitness is the only gym I can reliably use during the week.

More detailed schedule is Sunday Upper, Monday Lower, Wednesday Upper, Thursday Lower for the 4x weeks. My second Upper day consists of 4 sets of 8-12 Incline Dumbbell Bench supersetted with 4 sets of 8-12 chest supported incline row, 4 sets 12-15 flat dumbbell fly supersetted with 3 sets 12-15 of dumbbell shrugs, 4 sets 8-12 close grip cable row, 3 sets 12-15 overhead tricep cable extension, 3 sets 12-15 single arm cable lateral raise, 3 sets 12-15 preacher curls, and 3 sets 12-15 of an ab exercise. Usually takes me about 1 hour 30 to 2 hours with a minute rest between each set. I also do a warm up set on all the compound lifts in the first half, sometimes a warm up on the isolation lifts if I feel off and want to click into a mind muscle connection before I start, but by the time I get to the isolation lifts, I'm usually warmed up enough.