At 4 months old it sounds like it was a hypoglicemic episode, but you also need to keep a watch for other possible complication that could seizures. Your description is of a seizure, but at this age it could have been just a low sugar spell, specially because it happen when the puppy was active for a while. Keep an eye to it, mention to your vet during your next visit and keep a good feed system to avoid any more low sugar episodes if this is the case. If it is something else it will happen again and some more investigation may be necessary sooner rather than later.
Blessings.
XOXO
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