When I attack with a shadow clone and banish a Unit I get 4 Assault. When I play Ride the Wind and banish another Unit, does he have Assault 8?

Yes — Assault 4 + Assault 4 = Assault 8.

Each time the Shadow Clone attacks, its "When I attack" trigger fires and grants a fresh Assault 4 for the turn. Those are two separate grants from two separate combats, and per Rule 807.2 Assault values from multiple sources sum. After the first combat resolves, Ride the Wind moves the clone into a new combat where it again gains the Attacker designation, so the trigger legally fires again (each "When I attack" fires once per combat — Rule 383.4.e.2.a — but these are two different combats).

Two things to keep in mind:

• You need a second unit in your trash to banish for the second trigger.
• The second battlefield must be occupied by enemy units (a real Combat). If Ride the Wind sends the clone to an empty battlefield, there's no Combat, no Attacker designation, and no trigger.