Can it be done? yes.

Would I advise it? No!

If your planes come in and go out as scheduled (always chancy), you'll touch down at 1510. By the time you make it through customs and immigration it could easily be an hour later. Your flight departs at 2010, and SEL recommends being at the airport 2 hours ahead of departure time. I've cut that short, but wouldn't be inclined to cut it shorter than an hour (to make sure you get through the check-in chaos and back through customs and immigration in time).

Really that leaves only 3 hours. Downtown Seoul is at least a half hour away by taxi, assuming no traffic problems (i.e., I think it was actually done once back in the '80s). So, if everything clicks like clockwork - not likely - you really have only 2 hours to see the city.

Better to extend the mileage run one day and really take a look at the city. It's worth it. Great Oriental palaces and shopping areas and lots more.