I have done the C25K multiple times and love it. I know there used to be a program where you started to increase your running to 4 days a week. Does anybody remember that one, or know of one? I want to either do that one or do some 10K training. Not sure if I'll do the Galloway or the Higdon method...or the One Hour Runner. Just not sure yet. The running bug has bitten me again and I'm wondering if I want to add another day to the program.
FYI, my next run is the big 25 min run. I'm just trying to plan what my next move will be.
I'd say the question is, are you training FOR something, or do you just want to run 4 days/wk? If the latter, then just do what most of us do, and run 4 days or 5 days, or whatever you like. Be careful not to increase your mileage too quickly, though. A lot of us probably base our individual runs/workouts loosely on training programs (some days being a long slow run, some being speedwork or hill repeats, some being "tempo" or medium-hard runs, etc.) but you don't absolutely have to follow a program. If you've done the 5k program many times, a 10k program certainly could be your next step, if you want to follow a specific program. But don't get too hung up on there being a magic formula for running once you've built up the fitness for it. Keep in mind that C25K is designed for people going from basically a sedentary lifestyle to running. If you love running, just do it!