If you are able to make the pergola into a walk- in run with the coop inside, the walls should be 2 metres high, and then a roof over that. Has the pergola got beams across the top? If so you could use them. I've used clear plastic corrugated sheets with a gutter for my run roof, which lets in light but helps to keep it dry. If you don't want a solid roof you should cover the top with the same mesh as the sides because foxes can climb in otherwise.
Opinions differ on what mesh to use - I've been pleased with the 1/2 kind because it excludes small vermin and the small size if the mesh prevents foxes biting into it. It comes in a thinner kind of wire than the 1", but you get just as much wire because there are more little squares making it up. Lots of people use the 1" size, which is fine, but if you go any bigger than that there's the risk that rats, mice or small birds can get in and possibly spread redmite or eat and contaminate food and water.
Just get a big bag of grit and shell and put out some in a heavy little pot, so they can have a nibble when they feel like it. They don't need much, just top it up now and then. Their layers pellets will contain plenty of calcium so the grit is the most important part, to help them grind up their food in their gizzards. Garlic in water is good but optional, like most supplements. Not essential but up to you. However, you should buy a pot of Flubenvet worming powder on the Internet, look around for best price, and treat them all a week of so after they arrive, once they've settled in and are all eating well. I've usually found that new birds go off their food a bit when in a new place as it's stressful for them, and as you add the powder to their pellets you want them to get the full dose. One pot will treat 20 hens, so 5 goes for yours, at 4-6 month intervals. You can buy Marriages pretreated pellets but you're not likely to get exactly as much as you want in a bag, and any left over will go out of date. The powder is easy to mix and has a good shelf life. For 4 hens you could mix 4 kilos to start with, ie 2 scoops of Flubenvet, and then mix another kilo if needed. They have to eat just treated pellets for 7 days, no extras. Have a look on Poultrykeeper for info on worms and worming.