Four weeks in your IPA was at 1.048? You don't mention where it started, but that's not a particularly good sign. Are you getting fermentation now in the second vessel?
The addition of dextrose won't do anything significant. It bumped your gravity up a point or two, but your yeast _should_ be able to take care of it no problem. That said, something is up with your yeast. That's the problem, not the dextrose.
Dextrose is just sugar. You add a bit at bottling to give the yeast a tiny bit more to eat so they make CO2 to carbonate your beer. I don't understand what you mean by "actual sugar levels are not known". That's what your hydrometer tells you.