I think that if you keep giving her both sides at your 4 feeds you should be fine.

Try introducing a second comfort item whilst you feed her to sleep at the moment - hopefully then she can start to associate sleep with both boobs and whatever the second item is.

The second option is to start to feed almost-to-sleep, so she has to do the last little bit of falling asleep herself. That way she can start to get the hang of it.