c'mon, lets be serious ;) bear spear definitely MUST be heavy to provide proper durability to not break down against charging bear, but it is not the macedonian Sarissa, which weighs 5 kilos, but is much much longer (5 meters) and have two metal fittings- one as spear head, and blade on the opposite side. 

With a bear spear, we have only spear head (okay, it must be solid, just like Jeremiah said 10 inches of cold steel and also a crosspiece) - maybe even in-game 0.5 kilo weigh and 2 scrap metals for forging is really under-estimated, but 0.75 and 4 scrap metals is quite reasonable- and shaft. Shaft for a bear spear should not be longer than 2 meters, because longer will cause much larger torque (you can check it yourself- shorter sticks/poles are much harder to break/bend in half than longer ones), so the shaft would have to be thicker than male arm to stand the sprains resulting from charging muscle mass ;) even this final in-game spear is about 1,5m long when showed on ground (after fixing in the forge). in such case- weight of shaft should give additional 0.75-1 kilo weight (keep in mind it is dry wood, it is not so heavy). Final weight of 1.5 - 2 kg of spear, dont you think it sounds reasonable? ;) And I think it would be not a drama for keeping in-game inventory weight balance. Game balance may be however endangered in resources needed for crafitng- but it depends also on how many of them will be needed (hard to predict, saplings for harvestings are about 1,5m in height, so 1 would be enough, but harvested have about 0,5 m in length, so up to 3 could be needed to craft a spear) and how quickly such a spear will be wearing off- the one in story mode seems to be indestructible, as quest item, but I dont suppose such a mechanics will be in survival mode, maybe it will be one-time, maybe like bow arrows 3-5 hits and breakdown. Maybe there will be additional potential spawn locations or even another kind of saplings, proper for only spear crafting ;) It would be quite interesting to introduce possibility of harvesting the spear shaft as one-piece solid wood from fallen trees (I mean whole healthy tree trunks, not the limbs, or hollowed, carious ones) with use of knife and hatchet/hacksaw :)



And of course- even if story-mode indestructible spear has to weigh those almost 4 kilos (because of indestructibility ;) it seems not so well considered to make it impossible to drop this item- world exploring with such mechanic is much harder, so it forces for pursuing the story-line, when it is forged. I know it is "unreal" but maybe it is reasonable to make it weightless as quest item.