Hello,
excessive masturbation does cause that, and don't worry, take some time off and be gentle to it, and it will become healthy again. You say you already see progress, so refrain for at least 2 weeks, and then try and find a balance with masturbation!
Yes. As you already mentioned, you didn't masturbate for 4 days and see a stronger erect penis. So keep it up...
Every man is different. Some men can masturbate all the time and still have healthy looking penis and some men should only do it once a day. You need to study your body and determine how often you should masturbate.