If you can get 4 bolts into a 4x4 vertically, AND the mount was designed for that bolt pattern AND you've checked the weight limits of the mount, you should be OK. With the TV against the wall, the torque on the bolts is very low. You mostly deal with shear in that situation. Once you start extending that TV away from the wall, the torque forces go up VERY fast. If you NEED an articulating arm mount, then you NEED a very solid anchoring system. TVs can really hurt people (kids and pets?) when they fall.
Ask yourself this: How much did the TV cost? If it falls because of an insecure mount, can you easily replace it? If the TV falls on a guest or a child or pet, can you afford THAT? Personally, I'd be taking that wall apart and beefing it up with 3/4" plywood backing at minimum that spans the studs.
Cut a rectangle of drywall out from between the studs. Rip 3/4" off of a 2x4, predrill holes, and lag a length of that ripped down 2x4 vertically to the 4x4 on each side of the opening. You should then be able to lag a piece of 3/4" plywood to the 2x4s between the 4x4s such that you can take that piece of drywall and put it back in the hole it came out of and it will be tight against the plywood backer board and you have minimal patching to do.