A lot of items (Warp scramblers, neuts, nosses and others) have identical stats to T2 while taking up less cpu and powergrid to fit. This how-ever means they are considerably more expensive than T2 and are often used of the ship won't fit otherwise (or you don't have the skills for the T2 mod yet.
Just keep in mind T2 guns are ALWAYS better than Meta 4 and even faction guns because with weapon spec training to 4 you get an 8% damage increase on T2 guns over Meta 4 with same skills + you can use T2 ammo in them (the use of these is mostly situational though, largest boon is the damage and in some cases T2 ammo, say Amarr really isn't worth flying in PvP unless you can fit Scorch ammo into your pulses)
So in short Meta 4 if the items have same stats and you have squize in those last few PG and/or CPU for the fit, or you can't fit the T2 item yet because of skills. T2 are almost always cheaper.
An exception to this is the armor plates (EA 1600mm reinforced rolled tungsten plates), the Meta 4 is easier to fit and on top of that is better stat wise than the T2 (less mass addition to your ship = ship moves faster)