This is my guess. It sounds like what you are saying is that the transmission is staying in neutral during the 3rd and 4th gear operation. Have someone operate the gear shift through all 5 speeds. While you're under the vehicle, observe the shift lever operation when shifting from 2nd to 3rd, from 3rd to 4th and from 4th to fifth. I'm thinking that each cable has 3 positions (forward, middle, back). When in any forward gear, one or both of the cables must be away from the middle position (the middle position should be neutral). If both are in the middle position when in 3rd or 4th, then obviously to me at least, the cable that selects 3rd & 4th gear isn't moving the shift lever on the transmission. Check the adjustment and position of that cable at the shift lever and the transmission. Maybe the lever and cable are moving but the shaft on the transmission isn't. Maybe the new cable itself is defective. It's hard to troubleshoot from a distance, so I'm only offering a suggestion. Whenever I work on something and it doesn't work, my first and only suspicion is that I did something incorrectly and it wasn't necessarily caused by a failure.