12 v 100 watt panels that would be what, 8.x amps, Times 4 would be about 37 amps, bit much for 10 ga, but those panels hardly ever deliver full output, if ever. so you can at the most expect around 15-20 amps out of 'em, well within 10 ga,, however for solar panels you want heavier wire to reduce voltage loss.
What I would do
See if that wire will move up and down in the path. If so use it like "Fish tape" and pull heavier wire if you can. If it's stapled.. Oh well.. You tried
Yes you can use it, but a larger wire will give better charging, or double the wire, However double the wire will need a bigger hole. (Or a Much bigger hole).