Unfortunately there are no Level 4 ship gear that retails for credits. Your best bet is to either get the badges or look on the Exchange for them.
You can, however, get Mk 3 and 5 gear from SB Sierra in Alpha Centauri or Mk 5 and 7 gear from DS9.
The pattern seems to be that you can get the lower tier gear for your rank (i.e. Mk 3, 5, 7) from vendors but higher tier stuff (Mk 4, 6, 8) have to be from drops, quest rewards, or badge exchanges.
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You can, however, get Mk 3 and 5 gear from SB Sierra in Alpha Centauri or Mk 5 and 7 gear from DS9.
The pattern seems to be that you can get the lower tier gear for your rank (i.e. Mk 3, 5, 7) from vendors but higher tier stuff (Mk 4, 6, 8) have to be from drops, quest rewards, or badge exchanges.