you'll be fine letting your other batch sit in the secondary for another 3 or 4 weeks.
My batches typically don't get into a keg until a month after brew day, and sometimes as long as 2 months after brew day.
As for the longevity of the beers in a keg... as long as you sanitize and transfer carefully and properly, it will keep (meaning "not spoil" for a very very very long time).
Agressively hoppy beers tend to lose some of their hop character. but the beer will still be drinkable.
Ground Fault Brewing Co.