Yeah was thinking some 4" tapcons too. Counter bore the 4x4 2" down so the tapcon gets into slab 2". Space about every 18" or so (whatever works for the overall length)Pick whatever works for you. Tapcons, lead shields, epoxy, they'll all work. You can even use a couple pegs of 1/2" rebar. It doesn't matter if the 4x will lift up because the load is always lateral.
I just laid the 4 x 4 on the floor. It doesn't move as the tires are taller than it and they tend to push it down more than move it.
But that's inside a garage and might not work as well outside.
I'd look at some tar/roof sealant as a n adhesive of sorts.
Railroad tie might be an option. Many lumber and rural suppliers have them for $20 or less. Since the weigh far more than a 4 x, they are less likely to move, but they might be too high for smaller vehicles.
I'm generally opposed to drilling holes in concrete outdoors. Makes for more rapid failure due to water infiltration and freezing/spalling.
If you hammer some nails top and cut the heads off of 'em you'll be sure nobody bumps into them unless they really feel like it.At 8 1/2 feet, they're more than wide enough for most vehicles. Averaging around 200 pounds, they won't move unless you really want them to.