I have a withdrawal of 3 sats that with fee shows 4.001 sats. No problem there.

But the wallet balance only shows a whole number, and was reduced by 5 sats after this transaction.

I'm just curious, did this 4.001 spend take 5 sats from my balance, or does my balance still include the 0.999 sats, but the balance just doesn't display it (i.e., truncates any decimal amounts)?

4 sats \ 0 replies \ @k00b 4 Jan

It truncates it. Internally your balance is in millisats