my son had some odd habits, sitting "funny" tucking his legs up under himself sometimes, doing weird exercises etc. I always said he could break his leg
sitting a chair!

It doesn't take much to pull a tendon, and then they take a long time to heal.

Don't forget magnesium. Potassium is obvious, but low magnesium really leads
to cramps.

Some testicular pain can radiate down a leg, but 4 is rather young. The little
support tendons, can twist, and break and the testicle can then move when it shouldn't and then a sharp pain results. A doctor can verify this.
A friend of my son had it, when he was a teen. Doesn't even remember how it happened.
Also boys can have an inguinal hernia...only a doctor can evaluate this. It causes intermittent groin pain. You may or may
not see a bulge there when the intestine moves.

The bike may be a factor, combined with low magnesium. My mag thread has links for foods high in mag. Sitting in a warm bath of epsom salts usually relaxes muscle cramping.

Has he had an immunization lately? They give that in the thigh you know often...and that can hurt like the dickens.

If it continues past a couple of days, I'd get it checked out ASAP.