Osage Orange is legendary for surviving soil contact. Black locust is almost as good. See if you can find either one. Nothing wrong with using untreated douglas fir (or whatever they have where you live) in thick dimensions. It will last for a few years or even 5 years. Then you rebuild. Don't forget to staple hardware cloth to the bottom if you have gophers where you live. I also use galvanized metal straps on the corners of mine to keep the edges from separating. I mostly make by beds out of 2x6 douglas fir. I have one or two made out of 2x6 redwood for comparison, but they are quite new. If you use 4x4's they will take longer to rot out, but it will be more expensive and heavier to deal with. I design my beds to be exactly 4 feet wide on the outside so that I can use 4 foot wide rolls of hardware cloth. I use 8 foot lumber for the long edges. In my climate, with dry summers, I expect the fir to last around 5 years, but I don't yet have 5 years of experience with them yet, so we shall see. I expect to use the hardware cloth for at least 10 years, which is good, because it is expensive! Oregon state did a study with many types of wood fenceposts. There are a lot of rot resistant woods out there. But Osage Orange was the only one that had zero failures during the whole 50 years they did the study. In other words, 100 percent of the Osage Orange fence posts they installed in the beginning of the study were still serviceable 50 years later. I've added a link to the study in case it is of interest. Obviously the climate is different from yours where the study was done. McKenzie Here is a link that might be useful: Oregon State fencepost study

It's advised to not use PT wood because there are safer alternatives. That said, the stuff isn't a deal breaker for your garden. Once built the biggest real danger is putting your hands all over the stuff while harvesting...contact stuff. There isn't much to leach into the fruits even in the most toxic of PT lumbers. Plants generally don't pass on much soil toxins because they have no use for their uptake and very little makes it to the actual fruits. Like the above poster said...if you bought your PT lumber recently there's an even larger chance that it's the safer stuff. A lot of the issues with old PT lumber involved people putting their hands all over the stuff or laying produce on it to rest. Even then, a good washing takes care of most all of that. It's the arsenic in the wood that was a huge issue with the last generation PT wood and how you could accumulate arsenic levels on skins just by rubbing your hands across it. A lot of the "panic" came from how much PT lumber is used in kid's playground equipment.

kali- Part of the issue is what it's treated with and what the concentration is. Generally, current pressure treatment methods are considerably less toxic than they used to be, but any time you're using chemicals to kill/deter bugs from eating the wood, you're still using something that -- in my opinion -- is not ideal in close proximity to a vegetable production area.

The old CCA PT wood was known to leach Arsenic especially as it got older and because of that and that children could get that Arsenic on their hands and potentially lick if off is why that CCA PT wood was banned some years back. You must make direct contact with the wood and any Arsenic that would be washed off the wood and into the soil will accumulate near those posts, it does not migrate far in the soil and plants usually do not uptake enough to make them hazardous to you.