if Shadow Clone is in a Showdown (not combat showdown) and use his ability to get 4 Assust. He becomes a 4 might or 0?

It stays 0 — Assault does nothing in a non-combat showdown.

Shadow Clone is a 0 Might token, and Assault 4 means "+4 Might while I am an attacker" (Rule 807.1.c). In a non-combat showdown there are no Attacker/Defender designations at all, so Assault never applies (Rule 807.1.d; FAQ #3345).

Also, the clone's own ability can't even fire there — it's a "when I attack" trigger, and the clone never attacks in a non-combat showdown. If it later becomes an attacker in a real combat, its Might would be 4 (0 + 4) while the Attacker designation lasts.