I never made it to a 4 hour schedule and was more like 3-3.5 right up until solids which we introduced a week before 6 months. I read a lot about why the recommendations were changed to 6 months with the focus on waiting until 6 months for exclusively breastfed babies in particular. I looked at why the WHO recommendations shifted and it's also worth bearing in mind that so many national health organisations changed from saying 4-6 months to 6 months - these are conservative organisations who do not take such changes lightly. There will be individual babies who are recommended to start solids earlier - e.g due to reflux or health concerns. However for the vast majority of babies the new recommendations are a good fit.
I would say that she's obviously doing really well and is curious about the world around her but even just a few more weeks delay may benefit her in the long term. The 'signs of readiness' discussion comes from the time when we were keen to start solids asap - now it's ensuring the baby's digestive tract has fully matured.
I would say that a useful step is to talk to your doctor about supplements that can increase milk supply - assuming you have received advice on supply from LCs and really exhausted all the usual avenues. I recognise your photo but I can't remember what your bf history is - sorry.
Starting early solids can have a bunch of effects but one could be a decrease in demand for milk due to feeling full on solids which are significantly less nutritionally valuable and the nutrients they contain are harder for a young baby to absorb. This may decrease your supply further.
I know it must seem disheartening to feel you are not meeting her needs but I would advocate solids as an absolute last resort - formula milk is a preference if you need to supplement with something other than your milk (and please bear in mind I am training to be a bf counsellor and recommending formula isn't my usual course of action!). Post again on the bf board and let's really try and talk it all through and see if there's an avenue you haven't explored.
If she's just 4 months now you are talking about just a few more weeks - feed more frequently, talk to your doctor, do some reading. Those few weeks will seem like a blip when you look back but if she did have a reaction to early solids (which might not reveal itself for years) it will be worth it.
JMO. I know other people will say they started their babies early with no regrets but the WHO,AAP, Health Canada, UK NHS folks must be worth paying some attention to.