prepare_release_branch: Expand `suffixes` to include `stable4`
prepare_release_branch: Expand `suffixes` to include `stable4`

Signed-off-by: Stephen Augustus <0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33@auggie.dev>

kubernetes/test-infra,kubernetes/test-infra,kubernetes/test-infra,kubernetes/test-infra,jessfraz/test-infra,kubernetes/test-infra,jessfraz/test-infra,jessfraz/test-infra,jessfraz/test-infra,jessfraz/test-infra,jessfraz/test-infra,kubernetes/test-infra