Four surgeries is a lot. Yes, the left eye is lower than the right eye. So, I would say the most important thing to talk to your surgeon about is "expectations". Most reasonable oculoplastic surgeons will spend a fair amount of time explaining realistic expectations, especially for congenital ptosis, because the outcomes are very different than for regular ptosis. That being said, it might be worth asking your doctor about a Mullerectomy on the left side. Also, be sure to ask about risks of dryness and incomplete eyelid closure after a 5th surgery.
The reality is that congenital ptosis is an enormously difficult and complex issue.
To have an upper eyelid crease as you do, you must have more than 8 mm of levator function. Sling surgery is very unsatisfactory but occasionally necessary when there is less than 8 mm of levator function. It should not be a serious consideration for you. The question is could you be improved with additional surgery? The answer is not necessarily. I would be extraordinarily careful seeking out more surgery. It is one thing to promise an outcome and an entirely another to delivery that result. I would not be in a hurry to have more surgery.