The problem with most fourtrak's is they have normally worked very hard throughtout there life, and can be an ongoing project if you see what i mean.
If you manage to find a good one chances are it's reflected in the screen price. very good off road, but not really any better than a sportrak. The 2.8 diesel has plenty of tug if your towing, otherwise it's fairly lazy.
Both 4trak's and sporty's can be a rust hole true, then again that can be said for zuk's and landy's too!
Having owned both in the past it's hard to say, but id' go with a sporty, mainly cos you can get more motor for your money, and in better nick if you choose well. The 1.6 engine has way more omph than a vitara or a SJ. If you can go for a EFI model- more speed & more mpg. I use to get between 25 & 30mpg from my sporty, depending on my right foot.
Other than that Daihatsu in general make some solid 4x4's and with a couple of mod's and some Mt tyres a outstanding little off roader for not much money.
Kind Regards
Phil