Doesn't matter. When I weld 4130N to mild steel, I use ER70S-2 filler. The joint is only going to be as strong as the mild steel anyway, and if you used 4130N filler, you'd have to post-weld-heat-treat. So just go for it, you'll be fine whether it's moly or mild steel.
We use Cr/Moly basically everyday on sprintcar chassis' repairs here in Australia and if your tubing has a seam in it it almost certainly isn't Cr/Moly. All the Cr/Moly tubing we use is seamless and is usually printed on the tube with removable marker something along the lines of "4130 grade Mil spec condition N"
Even if it was Moly, I'd still use it because it's a much nicer material than normal steel tubing.
Regards Andrew.