Here is my solution for 4 kayaks outside. In the corner of my yard, I dropped two PT 4x4's in the ground with Quickrete. I used some hanger brackets from Home Despot, each of which is supposed to be able to handle 50 lbs. Over the bottoms of the hanger brackets, I used some pool noodle foam, which does naturally conform to the shape of the boat. On each side, I also have the Riverside 2-kayak hanger straps. A bit of redundancy, but should a good wind come along, the straps will help keep the yaks in place on the brackets. Also, should a bracket fail for some weird reason, my kayaks are not going to fall.
The reason for the extra cross-beam on top is that I built the rack when I only had three kayaks, and I needed to extend it further out (for the strapping) when I got my Perception 16.5' boat. Rather than replace the existing top post, I just lag-bolted an 8' 4x4 on the existing rack.
Each kayak is locked to the rack with a plastic-coated steel cable and a padlock. I just bought a pack of 4 padlocks, so there's only one key. Though this pic doesn't show it, all three of the sit-insides have a Seals cockpit cover to keep out critters. Behind the boats, in a deck box, I keep most of my accessories and gear, excepting paddles. It too is locked off with a steel cable.
I had to go online to find a good-fitting tarp to cover it all up and keep the sun off the boats. I went with a green, heavy-duty, UV-protective version made for outside storage. Tarping everything at once was/is a lot easier than tarping the boats individually. The tarp fits durn near perfectly, and I just use tent stakes in the eyelets to keep it secure.