Whilst there wouldn't be anything wrong with that approach, there isn't anything wrong with what the OP is doing either.
OP, if your 4 year old is happy to play alone for a bit in a safe environment, that is fine.
It also gives you chance to spend dedicated 121 time with your youngest.
I blame social media for the fact that mothers feel guilty about just about everything.
I agree. There is a lot of parenting 'advice' and guidance out there but that's all it is. Advice and guidance. Ultimately, you have to decide what works best for your family in your circumstances.
It's also worth remembering that there are other factors to consider. House size for a start.
If the OP lives in a vast sprawling mansion and being downstairs might as well be in another wing of the house, concerns are fair enough. But if she, like me for example, lives in a 2 bed terraced house then downstairs is 5 seconds away and effectively in the next room.
And, as others have said, the child's personality makes a big difference. I could and left one of mine this way because he was completely compliant and wouldn't move from a given spot if told not to. The other, I couldn't because she'd have likely decided to remodel the house in my absence. I'd have got downstairs to find her taking a lump hammer to a supporting wall and telling me she'd decided we needed an open plan kitchen/dining room.
Presumably, the OP knows what sort of child she has and wouldn't be doing it if it felt dangerous.