For cheap price, the Moras are tough to beat. Check out the Swedish Army Knife (SWAK) and/or the tri-flex steel Craftsman or Tradesman from
Ragnar's Ragweed Forge as the others recommended. At the price of Mora knives, for less than your $70 limit you can get two or three & stash the lightweight little fellas throughout your gear with bucks left over.
However, my primary recommendation is another vote for the Swamp Rat Knifeworks blades. For just a few bucks more than your $70 limit I recommend the Howling Rat, even though it has a 4.5" blade. I have one and it is a bombproof-tough blade. The handle ergo's are very good. Resiprene-C handle material doesn't get "slippery when wet" & is impervious to many chemicals (including DEET which eats kraton handles badly). The blade on the Howling Rat has a decent slicing geometry. The blade is made from 3/16" thick hi-carbon steel. It's not as optimized for slicing as the Moras, but it does cut well and is many times more durable. As noted, the Swamp Rat warranty is top notch, i.e. "If you break it, we'll replace it." Check out Cliff Stamp's review of the Howler here:
http://www.physics.mun.ca/~sstamp/knives/howling_rat.html
Alternatively, check out the Swamp Rat Bandicoot or Bog Dog models, both with 4.5" blades. The 'coot has a narrower blade (spine-to-edge dimension) in their SR101 steel, which is a tweeked version of 52100 hi-carbon. The Bog Dog is 1/8" thick D2 with an amazing amount of toughness for a stainless type steel. Check out the pics & report of Swamp Rat D2 blade torture testing here:
http://www.bladeforums.com/forums/showthread.php?t=322561
Becker Knife & Tool model BK-10 Crewman from Camillus is another decently tough & versatile blade. But its blade is 5.5" long, so it too is a bit longer than your stated 4" blade specification. The BK-10 costs about $40 from
Bayou LaFourche Knifeworks.
A few other makers whose blades might suit your size & price range:
- Kabar
- Ontario
- Cold Steel, especially their factory seconds
- Ranger Knives from Justin Gingrich