Woodworking – U Brace/Bracket to Hold Vertical 2×4 Over Wood Floor

How old is the wood? If we're talking 70 years or so, definitely don't burn that stuff as that'd be old growth wood which you just can't get anymore (easily). I'd even have a hunch that a 70 year old 2x4 might carry as much of a load if not more than a modern cheap pine 2x6.

All that said, as you state, it's not treated lumber. Unless it is cedar, you likely won't get much life out of it as a decking material--unless you have plans to refinish it each and every year.

The 2x4 also wouldn't make for a sufficient floor joist, so you'd have to use new structural lumber for that anyways.

If it's old lumber, I'd consider saving it for another construction project, or donate it to your local re-use center or habitat for humanity. If it's not old lumber, I'd use it for the fire pit.

You call it a patio, so I assume it is not a wood deck (which would be easier to add a railing). I'd avoid attaching it to the house though it seems that the other end of the patio roof may be tied to the house already. Consider driving a super spike into the ground at each end to hold a post that will be the ends of the railing. At a 10 foot span, you'll want one in the center of those two as well. If driving the spike just off the patio won't work, then remove a patio stone and drive the spike where you need it. You can break/ cut the patio stone to fit around the post once its in place.