I'd try making it 8 frames for a generic run. Check out this great reference image (from idleworm.com)
Right now, your four frames are super-high-point, contact, passing, and highpoint. When the legs are at full extension they should be nearing the ground, like in the second frame you have. Then, the weight of the body will make the extended leg bend as the body passes over it. Also, don't be lazy and copy the first four frames, make 8 individual frames with care for the best result.
Of course there is a lot of variation, and a run does not need to be exactly 8 frames, and does not need to follow this formula. But, if you're just starting out and want something that looks good, I'd say stick with this until you are more skilled.