Not to be a nitpick Nancy but the Double-T was introduced in The Lost and Damned. You're probably thinking of the NRG 900, as the Double-T was nowhere to be found in GTAIV. UNLESS you're just referring to the DLC's as 'GTA4' as a whole then my bad.
Yeah I f*cked that whole sentence up, it's my bad. =P
On that note, the NRG-900 would be cool to see in GTAV too, but would that make too many sports bikes?