There are several options available and each ones has pros and cons:
Filter Pump: These are large and relatively good for travel inside the U.S. but many of them don’t filter out diseases found in water abroad (ie tropical countries). They’re also a relatively expensive and maintenance intensive option. One place filters can shine is gravity filters used in large base camps at remote locations where there are many fewer water contaminants. These can filter large volumes of water and supply base camps with a regular source of safe drinking water.
Iodine tablets: These are safe and easy to use but they have their down sides. First in humid conditions as those found in the rain forest the tablets can melt together some what making them useless or the lids can rust shut, I’ve seen both happen. Also, the tablets run out relatively quickly. Traditional iodine tablets also leave your water with a bad taste. Tablets are a viable option for a small party on a short trip and are by far the most inexpensive system.
UV purification: These devices are becoming more and more popular. They’re a little pricey but they’re effective and easy to use, faster than iodine and easier than a filter. Still, like any electronic device they can fail and need batteries. I would only recommend them if you have a back up or are travelling only where bottled beverages are commonly available.
Polar Pure: This is what I would personally recommend. It’s iodine but done differently, it leaves no taste and it’s just as safe as tablets. It also lasts much longer, the iodine crystals don’t degrade in humidity and the plastic lid doesn’t rust shut. It’s a bit more expensive than traditional iodine and if you’re going on a 4 day trip, it’s probably not worth the cost.
Boiling: This is the old fashioned, tried and true way to purify your water: just boil it until everything is dead (bacteria, etc.) You can’t go wrong if you boil your water long enough and it requires no special equipment beyond what you use for cooking.
Bottom line is: It’s best to purify your water but if for some reason you’re lost for a long time in the wild and have no way to purify water, remember this: You’ll probably die of dehydration long before you will or parasites or bacteria and most water borne diseases can be treated. Don’t die from fear of drinking contaminated water. Also, when in doubt, don’t take my word for it, check with the CDC.