You can. I have, but I think in my case it made sense: I was moving, and needed the money for the 23 hours that I would own 2 houses. Longer if you wish due to banking transactions, but you get the idea. I think it was $50 for the loan origination, and the money I had out was not earning interest for about 4 weeks total. A personal loan would have cost me ___?I think you can take a loan against a 401K, but I'm not going to do that.
I was contemplating taking a small loan recently for car buying (like less than 5% of my 401k holdings) but one of the catches is, if you change jobs, you have to repay the loan back very quickly--or risk it being a withdrawal and then paying a 10% penalty, then ordinary income taxes on it. Also, at least for my 401k plan, I cannot pay extra each month, I can only make the scheduled payment, or pay off fully. Lots of little catches, but, it probably makes sense, some of the time. [Also: loans are limited to 50% of the 401k value or up to $50k, whichever is the smaller number.]
For the 401k loan, you do pay interest, but it goes back to yourself, so that is an upshot--but that is money you pay, not earn, so nothing really gained here. Really should be one of the last arrows in one's quiver of dealing with money issues in life.