Is there a cmd in Stata to generate a variable with custom observations?

I want to create a variable with continuous observations( note : the observations should be of same length : specifically (0001 - 8000).

I want this in order to create a UID by merging this variable with other identifying variable. I have tried using (egen var = _n).But the observations are of unequal length(e.g : 1 - 8000).

Any help on the this would be really appreciated.

Thanks