Big Jandals.
I still have days like that....
The fact that your are concerned means you aren't doing anything wrong. She's still little, and trust me, in 20years, she's not going to hold it against you.
I had DD at CC a couple of days when DS was small, but we had to taper it off due to $$$ and now we've moved. I seriously cannot wait for her to start preschool again in a month or so.
In the mornings when DS is a bit more high maintenance while I'm doing all the things I have to do I set up a couple of activities just for her (lego, textas, puzzles, littlest pet shop - stuff he can't play with) on her table and the beds in her bedroom. WHile I'm pottering she can still natter to me and DS isn't in her face.
While he sleeps, she's allowed to pick what she wants to do.
FWIW there is another thread today on 4yos and the trial they can be as well, so please don't feel like you're doing a bad job.