How safe is the stock market?

[Courtesy Flush]

I apologize for the ultra noob question, but better safe than sorry so I need to ask.

I've had most of my money sitting in online savings accounts for the last few years and have been told I am a fool for not investing it. So I finally decided I am going to buy a bunch of ETF shares as recommended in the "Investing for trout 101" thread in the FAQ here.

My question is, can I feel safe investing all of my savings into this? Is there any realistic risk of a market crash that I should be concerned with? Is there any reason I'd want to leave a portion of my money in government insured savings accounts earning less interest?

Also, a bonus question for anyone friendly enough to advise me. I figure I will invest in all 4 of the suggested ETFs (DIA, QQQ, SPY & VTI) and possibly even more. Could anyone recommend which %s I might want to put into each of them, or if I'd have any reason to want to avoid one or more? If not I'll just go 25% for them all like the donk I am.

Thanks for the help guys!