There are multiple methods you should consider:
I assume you're trying to downsample a signal.
If that is the case, consider the "resample" funciton (documentation linked below).
Alternatively, you may use the "interp1" function (documentation linked below) to perform a simple interpolation on your 5034x1 vector and get a 1004x1 vector output.
If your vector represents some sort of signal value over time, resample is recommended over interp1.
Again, there are multiple ways to do this, depending on your requirement.