What was your recipe, yeast strain, and fermentation temperature?
Probably you have more to worry about from the bottling process and priming sugar restarting the fermentation of all that residual sugar and creating bottle bombs than you do from the beer not carbing.
There are three scenarios, here:
1. The yeast is actually dead. At four weeks since pitch, in a beer that's currently somewhere around 5.6% ABV, this is exceedingly unlikely.
2. The yeast finished up all the fermentable sugars and went dormant. This is possible if you did all-grain and mashed *REALLY* hot, or used a huge amount of unfermentable grains/adjuncts -- certainly possible, given you didn't post your recipe or mash schedule, but, we'd be talking mashing at 160° and/or many pounds of crystal/cara/etc malt.
3. The yeast quit early due to being too cold, wort not being well-oxygenated, being a slow-working lager strain, something like that, but will pick back up again after bottling, chew that 1.028 down to 1.018 or something, and your bottles will start exploding in a few weeks. This seems the most likely case.
If I were you, I'd try and jump-start that fermentation.
If it's an ale yeast you used, I'd warm the beer up to the low 70's or so, keep it there for a few days, rousing it once or twice a day, and see if I couldn't knock a few more gravity points off. I got my last batch down from 1.020 to 1.016 doing that for a week and a half or so.
Otherwise... I have no experience with lager year, or other ways of getting stuck fermentations going again, though I've heard pitching a starter at high krausen or racking the beer onto the yeast cake of another just-finished beer are supposed to work well...
