Momentum on a free kick

[voiceoflg]

K has been kicking high with the ball coming down around the R 30 yard line all night. R4 is camped at the 30 when on a kickoff, K kicks it over R4's head. R4 runs back to get it and picks it up on a bounce at his own 2 yard line. Momentum carries him into the end zone where the white hat immediately blows his whistle and signals touchback.

Is this the correct call? If not, then what? I'll give what was eventually ruled later.