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Since we're about to launch version 4 to Stable, set it as the default.
This CL also updates a few tests that implicitly use the default model
version.

Fixed: 1054056
Change-Id: Icb2d781279ac132019ab9bd6d433dbb627490713
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316335
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Bettina Dea <bdea@chromium.org>
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791403}