I just had new vinyl siding placed on my house. Where the outside lights had been, they placed vinyl master mounts. There were no junction boxes previously, so the wires just stick out of the small opening on the master mounts. Should I use 4" round pvc boxes before mounting my new lights? Unfortunately, there are no studs around the wire, so I am not sure how sturdy the round boxes will be once I attach them to the house. I purchased 1 1/2 boxes(depth) because that is the thickness of the master mounts. There are cutouts for 4 inch round boxes, and I assume that is what I should use.

Can you get to the back side where these will be mounted? There should be studs in the wall and you can mount a box on spanners between the studs.

Use either round or wall case type old work boxes for this problem. Old work boxes have wings that open larger than the box opening inside the wall to grip onto the inside surface of the wall material and squeeze until tight and support the box.