The only "negative" is that because the money goes in pre-tax, it will be taxed when you take it out. Of course, the idea is that when you are retired, your income (and therefore your tax bracket) are a lot lower. You also cannot withdraw from a 401k early without stiff penalties, so once it is in there, you should leave it there until retirement. But if you have a match, then you are basically being given free money, so you should definitely invest at least the 4%