What is making you think your supply has dropped, is it just the lack of nappies, or anything else? There is a normal switch at around 3 or 4 months where you're moving from hormonal based post birth overenthusiastic supply to demand based, tailored supply. This means that you'll stop getting bigger, full-feeling breasts between feeds - it's normal for them to feel soft all of the time - and you might stop leaking. Women are not often told about this, and it can be mistaken for low supply. Sometimes the baby gets frustrated at the slower flow of milk because when your breasts are engorged, the milk can almost gush out very fast especially at the start of a feed, so to simulate the faster flow, you can try doing breast compressions which means squeezing/pressing the breast as she's latched to increase the speed of the flow.
At four months babies often become fussy at the breast, too. This is often due to them becoming more aware of their surroundings and suddenly looking around is much more interesting than trying to take a feed. Many babies also begin to wake more at night at this age too, sometimes thought of as a sleep "regression" and seem to switch to a pattern of more frequent smaller feeds, which can be a pain if they've been going longer, and baffling if you were expecting feeds to consistently spread out! You can help this by choosing boring, unstimulating places to feed or perhaps (if your baby will tolerate it) using a nursing cover to block out distractions. You can also frequently switch sides which can help keep them interested and feeding for longer. It's fine to feed from each side more than once per feed.
Lastly teething can be an issue at this age. You could try feeling along her gums for any hard lumps which could indicate imminent teeth or try the granules you can buy or teething gels just before a feed. Just be careful not to give too much of the gels near the back of the mouth. If she's pulling off the breast and crying then it can indicate pain in the gums, but it's all a bit of a guessing game.
FWIW, GPs do not have any breastfeeding-specific training. They can recognise important issues such as dehydration in newborns but I would not (personally) rely upon a GP's advice about breastfeeding itself. If your supply is low, then the best solution would be to use methods which are shown to increase supply such as switch feeding (frequent switching of sides), frequent nursing, skin to skin and possibly also added pumping sessions and/or use of galactogogues. Less frequent feeds and offering more formula are ways to reduce supply, not increase it, although top ups are sometimes necessary. Whatever you decide to do over the weekend though, any effects on your supply should be fixable, so focus on getting food into her however you can best manage. Certainly I wouldn't withhold feeds from her in an attempt to get her to take the bottle, that seems likely to exacerbate the problem, especially if you know she'll at least take something from the breast.
(Sorry, the site went down as I was trying to post, so I might have missed some posts!)