Actually vector can possibly push 4 with expanded hangar bays!
But four what getting the bonus?
Possibly 4 non heavy squadrons? Or a combination of some to push past those with heavy? Who knows. Whatever options you want at the time.
But I mean, pushing the envelope and all... Going from Speed 4 to Speed 5 is not a great addition - it is an addition, to be sure, on par with getting Speed 4 instead of Speed 3 out of Indy... Half the Squadrons you want to launch for an Alpha Strike at Speed 5... Are already Speed 5... And flinging TIE Fighters a way distance is just asking for them to be 3-hull-picked off...
In short.
Neither are popular.
But I still see Indy around almost 10:1 in comparison to Vector.
I was just merely adding the point that Vector can push 4 if it wanted to thats all.
But since we're looking at utility pushing 3 firesprays to keep up with Rhymer as the forth seems like a good idea.