I have 3 more-or-less full time starters, and I want to go with Starter A and B for the fourth slot, but that seems impossible, because Starter 4A/4B does is not a drop down option, and if i put them at Starter 1A/1B, I get a message that essentially states I cannot follow a regular starting spot with a tandem or an alternate A/B starter.

This makes no sense to me. Am I missing something? If not, WIS, please provide this option.

Yeah the Starter 4A/4B isn't an option. You could make a tandem 4A/4B, one starts, the other relieves. I do this with lefty/right combos sometimes.

Tandem 4A/4B is my preference as well, especially if one if left handed and the other is right handed, I assign more/less pitches based on the matchups.
It's also good if each pitcher is a different type, ie. A is low WHIP higher HR/9, B is higher WHIP low HR/9,

I would probably go tandem as well, but another option would be to set them as starters 4 and 5, while setting auto-rest at 99. Depending on how many innings they have, this might work out sort of close to what you were wanting in the first place. Otherwise, it's micromanaging the rotation every day to get your desired result.

You could also set it as starter 1, starter 2, starter 3a, starter 3b, starter 3b. It'll rotate your starter A/B between your 3 guys based on fatigue, with the lower OAV getting the nod if they're all at 100%

You got several good suggestions. Another way is to check in a couple times a week and manually change the #4 starter. I often adjust my rotations.

Thanks, all!