So in another thread (I forget which one, but I think it was one of the countless fighter argument threads) somebody mentioned that with a 2-handed weapon they would get a +4 on their CMB for trip attempts? I can't find this rule anywhere. Is this an actual thing or was somebody just mistaken?
There might be a weapon somewhere that grants a trip bonus. Alternately, there might be an ability which grants a trip bonus with a specific weapon. Without knowing what they're actually talking about, it's hard to say.
But no, there's no general bonus to trip attempts conferred by using a two-handed weapon over any other weapon.
If someone was using a +3 greataxe and had weapon focus (greataxe) then those would both apply to trip attempts made using that greataxe. (FAQ)
Several 2-h weapons get trip...and it's a standard that if you trip with a 2-h weapon, it's a +4, IIRC.
You recall incorrectly.
Trip: "You can use a trip weapon to make trip attacks. If you are tripped during your own trip attempt, you can drop the weapon to avoid being tripped."