If the fooling around was on day 8 after your period, and the doctor said you should wait a few more days than you did, there's a chance you could get pregnant, but calling it a "high" chance is an exaggeration.
A couple using no birth control that has unprotected sex at the right time of the month has only about a 20% chance of getting pregnant in a given month. If the implant was already in for 4 days, it will be giving you hormones. Though the doc said 7 days and you waited only 4, it's still pretty likely to have prevented ovulation, and if so, whether he had sperm in his pre-ejaculatory fluid or not wouldn't make any difference. And ovulation does not automatically mean pregnancy -- your chances won't be higher than couples actively trying to get pregnant have in a given month.
I realize 20% is not 0%, but try not to worry. Take an early-results pregnancy test in a couple of weeks and then forget it.
Hey there. Personally, I think you are probably good. He didn't ejaculate, you went from one nexplanon straight to the next and would guess that you still had some of the hormones in your system. Of course, not full proof and always best to follow what doctor says. Anniebrooke has given you great info. In 2.5 weeks, if you worry you may have gotten pregnant (again, seems unlikely but not able to completely rule out), you can take a urine home pregnancy test.