4 inches is pretty big for a folder. Bigger than you think it is. a Spyderco Tenacious is their best deal, it's about $34 and i think it would suit you very well. Its 3 3/8th inches, which is pretty perfect for most things a folding knife should do.
If you have your heart set on a 4 inch blade, there is the endura in your price range of "not to much". The rest are all pretty pricey. The endura is 3 3/4 inches, and retails around 60. For camping and fishing I think the tenacious would have a better edge geometry, though have yet to get my hands on one so far.
Ok, onto serrations! a well sharpened knife will outcut a serrated knife for pretty much every task, the only exception is for cutting some synthetic fibers, bread, and tomatoes, they are slightly better. but serrations have no place in the outdoors because you'll mostly be dealing with wood and other less fibrous matter, there are maybe 2 people who will disagree with me on that.
Combo edges are rather infuriating, with the exception of the kershaw serrations which are more scallop shaped. The serrated edge is too small to be of any use, and just hinders the plain edge portion by clinging onto material, and then snapping forward. you end up with a knife thats half knife, half useless.
Hope that helps, the topic comes up quite often and you can search and read a couple hundred pages of literature on the matter if you want.
Now onto a few tips to help us help you. I'm not trying to put you off here, just giving you some tips to exist happily with everybody. Lots of guys here are rather grouchy when it comes to bad grammar. I'm not too good myself by default, my brain skips over essential words on occasion, or sticks them in out of order. You'll notice i end up editing almost all of my posts. but give it a bit of effort to go back and check out for obvious things. Secondly fill out your profile so we know a bit about you, and who you are. Really helps when you're looking for advice. The thread tittle is important, We're all always willing to help you so you don't need to beg us in the title, put something regarding the topic at hand. Example "Good Budget Spyderco for outdoors?" with the suffice "Recommendation" instead of off topic. Once inside list some features you're looking for. You'll find you get a lot more reply's and help. Have a nice stay.
Edit: Shit, I see you're from the UK. I don't think you guys are allowed to import, or carry locking folding knives. I'm not sure if thats only for urban areas or not, looking up legality is up to you. It sucks, I know.