If this team has been good then by all means keep it. It seems pretty balanced, as long as Garen is tanking. For the last slot, Karthus or Jax might be a good addition.
A hybrid or caster would be optimal I think. Someone with a stun would be ideal, unless your Garen hasn't been very tanky, then you would do best with a tank. Maybe even Vlad wouldn't be a bad choice if you are worried about the other team getting Ninja Tabi/Wit's End as equips to shut your team down.

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Thanks for the help.
Soy Sauce FTW