What should I do about making up for the fasting of the past four years, the exact number of which I do not remember, knowing that I may have broken my fast on some days without a valid excuse during that period, and that in the first year I fasted the entire month without praying, and I was generally negligent in my prayers during those years?

You must make up for the days you missed in Ramadan, even if you were unaware of the ruling. If you are unsure about the number of days, you should make up for the number that you believe most likely clears your obligation. What you missed without a valid excuse requires repentance, and you owe expiation for delaying the make-up until the next Ramadan, unless you were unaware of the impermissibility of delaying.

As for missed prayers, the majority of scholars hold that they must be made up, just like forgotten prayers. However, some Shafi'is believe they are invalid and cannot be made up. Some scholars hold that one who intentionally abandons prayer is an apostate and is not required to make up what they missed if they return to Islam. The more cautious approach for you is to make up the prayers and strive to ascertain their number.