Do they abilities to reduce transwarp cd stack? How long will be my cd on transwarp if i use both? I need my cd to be 4 min or less, is it anyway possible ?

Yes. They do stack but additively i.e. ( time / ( 1 + (<total>/100) )

Might not need both...

I have a purple DOff doing the transwarp cooldown on a toon, and the cooldowns are 4 min, but if I'm not mistaken limited to only one slotted at a time (so no piling 5 DOffs to shrink cooldown). At the same time, I think that the DOff I have in question came from the colonial support mission in some sector, thinking Romulan space but not certain...

Actually, a single purple Astrometrics DOff and no other reducers will only take your transwarp cooldown to 5 minutes. Any astrometrics DOff will work, whether it's the one from the colonial chain or not, but at purple rarity, that's a +200% cooldown rate.

You probably just think it's 4 minutes because as soon as you transwarp, it goes from 5 minutes to 4:59 in...well, one second, and the game doesn't display the seconds unless it's under 1 minute so will just show 4 minutes from 4:59 until 3:59.