Well, +4 on the Custom Vehicle Trait is Design your own Vehicle with IS tech, +6 is the same with Clantech (I've never quite figured out if IS characters can jump over to the Clan side with enough points, since it says to use the "Appropriate Affiliation column.")
Of course, as far as I can tell, the +4 or +6 seems to let you design your own vehicle from scratch, which might be a bit excessive, and a bit overpriced, if you just want to swap a weapon.
What you could do is look at the Customization rules in StratOps, and then assign point values for each level. So a Class A Customization is a 100 point Trait, while a Class F is a 600 point quirk. You could then add a surcharge for going from IS tech to Clantech (or not, it just depends)
This would allow you to do a small customization without having to spend as much (Want a PPC swapped for a LPL? That's just 100 points), but since each customization would cost points, then if you got too crazy it might be better to just go with the +4 or +6.
Yeah, kind of rambling, I know, but customization has always been a bit of an oddity for me, since as far as I can tell simply modifying a mech and designing one from scratch costs the same amount...