I think hardwood's worth it if you are putting time in and expect it to last. My favorite piece of furniture I own is a desk I made out of cherry, and it's beautiful. My Dad has a few really nice cherry, walnut, and maple projects in his house, many of which still look great after 30 years. One thing to consider is that hardwood furniture generally remains in better condition - softwood tends to get dented more.
Also, if you mess up and cut your expensive hardwood wrong, you haven't necessarily wrecked your project. There are usually ways to cover mistakes - my cherry desk has a lovely inlay on the top that everyone thinks is an intentional design element but actually I accidentally ripped a big slash across the top when cutting breadboard ends with a tablesaw and subsequently used inlays to cover it up :)
If you are finding 4x4 to be pricey for posts - first of all, that seems enormous and overbuilt, would 3x3 work? Or, if you are set on 4x4 for design reasons, any reason you can't get 2x4, smooth the edges on a joiner, then glue two together to make 4x4s? As long as your wood didn't have crazy grain it wouldn't be too noticeable.