Is a 5 hour layover at SEA enough time to go out and do something in Seattle?

I agree with Debbie, you will have to hurry a bit, but its do-able! Taking the light rail including walking time will take about 45 minutes each way, so you'll have 2.5 hours of restaurant time and sightseeing. 

Restaurants tend to be pretty efficient downtown so you'll probably be able to be in and out in an hour and a half. My favorite restaurants near Pike Place Market are:

• Pink Door is an Italian restaurant with great food -- a local favorite.
• Maximilien Restaurant is a lovely French restaurant with a great patio area and happy hour.
• Matt's in The Market is another great restaurant near Pikes Place Market.
• You could also make the 10 minute walk down to the waterfront and have lunch at Six Seven Restaurant at the  Edgewater Hotel for great views while you eat. There are ton of other seafood restaurants on the waterfront as well. Elliott's Oyster House and Ivar's Acres of Clams come to mind. 

In your remaining hour of sightseeing you could:

• See the delightfully disgusting Seattle Gum Wall
• Walk through the shops of the Pike Place Market and eat some treats!
• Grab a coffee at the  Original Starbucks
• If you're up for a bit of a walk (10 mins or so from Pike Place), walk down to the Seattle Waterfront and check out the view.

Seattle is great! Definately worth the treck into town to check out a few things. Have fun!

Four free hours is cutting it a tad close, but if you don't mind making haste, then yes!  Lucky for you, SEA is very close to Downtown Seattle (I'm assuming that's the part of Seattle you'd like to visit!) and it's very convenient to take a cab or the train there.  Without traffic, a cab takes about 20 minutes and last I checked in 2012, it cost a flat rate of $40.  Like I mentioned, there's also a train that you can take from the airport to downtown, which is a lot more affordable.  The Link Light Rail costs less than $2 (last I checked in 2012, it was $1.75), but takes slightly longer, about 40 minutes.  Here's more info on how to get from the airport to downtown: https://www.trippy.com/question/Seattle-Best-way-to-get-to-Downtown-Seattle-from-SEA

If you don't have any particular place in mind to check out, I would suggest the very famous Pike Place Market.  There's a Link Light Rail station just four blocks away -- the Westlake Station.  You can browse the market, see the water, the flying fish market, eat at a seafood restaurant, visit the gum wall, drink some coffee (the very first ever Starbucks is there), pick up some souvenirs, and just take in the sights and sounds.  Should you have time, the area around the market is nice to peruse around, too.