Starcraft – Does replaying missions in Starcraft II earn extra credits

The campaign lists 26 total missions, though some of these are "A or B" missions. Though you may only complete one of these missions for credits / research / campaign credit, the other, alternate mission is available on the console on the bridge to be replayed any time you like for achievements, etc.

Given the nature of those "pick one" missions, I believe there are 29 total possible missions +- a bonus mission that is only unlocked during special circumstances. (Or there may be 26 missions +- the bonus mission, I'm unsure how the "Campaign progress" bar, x/26, accounts for the branching scenarios.)

It sounds to me like you've missed the secret mission "Piercing the Shroud" and the alternative choices on the Tosh/Nova Selendis/Hanson and Warfield/Tychus missions. For the latter three at least, you should be able to complete the missions by going to the mission archive and selecting them from the menu.

Alternatively -- did you do all the available missions before going to Char? If you didn't you'll need to start a new campaign to finish those -- once you go to Char, you can't go back and do any missions you may have missed.

It might be worth it to check your "Wings of Liberty " Achievement progress and list exactly which 4 missions you have yet to complete (The achievements that award Warfield, Mengsk, and uninfested Kerrigan portraits, for reference).

Edit: Okay, it looks like you missed the Zeratul Missions. After completing "The Dig" there is a new item in the Lab screen, in the lower left-hand corner that has "Crystal" or some such identifying it. Clicking it will let you take 4 Zeratul missions. It looks like you only did the first.