Congratulations for making it to the 4 mile mark! I found it to be extremely difficult to get beyond 3 miles, and then once I was able to conquer that mark, everything came easier!
I know how difficult this is for you, as I've been injured before and when you're making such great improvements like you have been, the last thing you want to do is have to take time off from running. But your body was telling you that you either need to slow down or you need new shoes or maybe something else--so listen. Rest as you have been and keep caring for it as you have but if it's still painful, do not run on it.
The best thing you can do is wait until the pain goes away (or if it is serious, I'd recommend you go see a doctor that is familiar with the running lifestyle) and then dive back in to where you feel comfortable. It may not be 4 miles...you may have to go back to 2-3 mile workouts for now until your endurance returns and you feel stronger.
No matter what, though, you should be very proud of what you've already accomplished and don't give up now. Keep your body healthy and strong and stay motivated! You're awesome for coming this far, not a lot of people can do what you've done!
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