To Cold Set or Not To Cold Set?
No need to cold-set.

I have a BSO with the same problem. The rear dropouts are about 6mm too close for the hub. I can just flex them apart and let the hub in.

The misalignment of the dropouts would be virtually nonexistent at 4mm flex.

If you remove your wheels often, and putting the wheel on is a pain, it might be worth cold-setting your frame. Other than that, don't bother.