Not a clear ruling, just opinion. I would say that the bonus is +4. The weapon "normally" has an enhancement bonus of +1, but the GMW spell *overrides* this and makes it +2. The Bane effect increases the enhancement bonus by 2, thereby making it +4.
It all depends on whether or not you see the Bane bonus as a "floating" bonus. For example, if you increased the enhancement bonus of the weapon from +1 to +4 (using craft feat, for example), the weapon is not +4/+3 versus Bane-type. The Bane effect adds to the enhancement making it +6 ultimately.
B