Refinance 15 year into 30 year on Primary Residence?

[Eric H.]

I currently have a 15 year mortgage at 3.75% on my primary mortgage and am 3 years into paying the mortgage down.  I am considering refinancing to a 30 year mortgage in order to lower my payment and increase my available cash for investing.  The 30 year mortgage will be a higher interest rate ~4% but my monthly payment will go down by $700 - $800 and I will be investing all the extra cash flow.  Another plus of refinancing is in the event that my wife or I lose our jobs in the near future then we have $700 to $800 less a month in that we have to worry about.  With either mortgage we don't plan on riding out the entire loan and in 5-8 years will probably start hammering the balance down.

What is everyone's opinion on this?  Would you refinance if you were in my shoes or keep the 15 year mortgage?