The not sucking I'm not sure what to do, just wanted to say my DD2 did this. We don't know if there was an underlying cause (very possible with breathing issues that were picked up later) or if that was just her thing but every feed until about 8 months was on off on off on off. In the early days she was just too tired and would run out of puff (IUGR baby) but then my supply came in with full force and after awhile I knew she wasn't too tired any more, but she continued with the on off thing. So I guess I'm saying that the continual stopping might just be how it is at the moment and maybe for the time being work on other issues and see how this one pans out once baby is a bit older and stronger?

As for the formula top ups, I had to do them as a result of the IUGR and it really is the hardest thing to judge how much and when because as you say you have no idea how much they get from the breast to know how much to top up!?! So what I was doing on leaving hospital (as directed by my LC) was every feed went BF, EBM or formula top up on demand - as in I always offered the EBM/formula and she either took it or didn't, and she dictated how much she took - then expressing (in that order). Feeds were every 3 hours around the clock when still it hospital but I went back to demand feeding pretty soon after getting home and she just stuck with 2-3 hours from then on. I found after a few days at home she didn't want any top ups so I gradually stopped offering and stopped expressing.

When you say she is feeding every 4 hours is that a routine thing or baby is demanding every 4 hours? If it is a routine think I would maybe try cutting it down to 3 hours and see if that makes any difference to the top ups the baby requires.
If the baby is demanding every 4 hours I'm sitting on the fence as to what to do. When they got me to routinely wake and feed DD2 every 3 hours it felt so unnatural and her and I were both against it (!) but in hindsight it did help us through our crappy feeding start so that tells me the idea has merit (especially with a tired IUGR baby), but on the other hand I am always going to have reservations about interfering with the supply demand thing that works so well. I guess she could always give it a try (waking and feeding every 3 hours) for a few days and if it doesn't change anything go back to demand feeding with top ups?