Ideas for approximately 4 hours in London

Depends on your interests. The area straight south of the Station (less than 2 miles south but don't walk this part, you don't have time to do that), the area bounded roughly by Trafalgar Square, Buckingham Palace, Parliament, and back up Whitehall to Trafalgar Square--that area has these within it:
National Gallery of Art (free admission so a quick visit works, if you know specifically what you want to see)
Buckingham Palace (no time to go inside)
Parliament (no time to go inside)
Horse Guards
St. James Park area
and much more.

Due to your time constraints, the above would be (except for the National Gallery) mostly a walking tour.
And not a bad walking tour!

We have some London contributors here but they're probably asleep by now. Look for their suggestions tomorrow.

Kent
Thank you for the suggestions. They are appreciated.
JoAnn

Arrive and hit the ground running. Believe the changing of the Guards is at 11am. http://www.changing-the-guard.com/dates-times.html Crowds start queuing up a minimum of an hour early and it's Summer so probably more. You only have 4 hours. I'd skip the ceremony and try this approach to seeing London. Buy a day pass. Use the tube. Travel from King's Cross/St. Pancras on the Victoria line to Green Park. Exit and take the Jubilee Line to Westminster. Exit and follow Signage to Exit 1. Walk outside. Turn right and climb up the stairs. Start your exploration there at Parliament Square. (Big Ben, the London Eye across the Thames, Parliament, Westminster Abbey) Snap pictures. Walk up Whitehall (Horse Guards, #10 Downing street (well guarded and behind gates), are on route) to Trafalgar Square. Coffee and breakfast in the Crypt Cafe of St. Martin's of the Field or if too crowded the cafes in the National Gallery are nice. Museum is free. http://www.nga.gov/content/ngaweb/visit/cafe.html. Nourished, walk past the National Gallery up and around the block to Leicester Square. A few more blocks to the Piccadilly Circus tube station, a ride on the Piccadilly Line to Hyde Park Corner. Nice glimpse of Hyde Park as you walk over to Buckingham Palace. Down the tree lined Mall and over a few blocks to Her Majesty's Theatre. Tube back to King's Cross/St. Pancras after the performance. Might be a bit ambitious but all you can do is try. Parliament Square at least.

Nice post from Claudia. She mentions St. Martins Crypt Cafe, where you can eat with dead people in the crypt. We ate there twice a few weeks ago. Those under the crypt are very quiet.

If I had just 4 hours, I'd go to Victoria & Albert Museum for a couple of them. It was one of my favorite places in London. I'd also opt to see Big Ben & Parliament, London Eye (not a ride just snap a few pics) and Westminster Abbey. Oh I guess that would all take more than 4 hours! Good luck whatever you decide to try!

Thanks for all your wonderful replies. I'll do more research this weekend based on your suggestions. I am amazed by the responses on this site. Wishing you all great travels.
JoAnn

After just spending a week there, I personally would just walk around and see Buckingham Palace and just get a look around London. It will be hard to spend a lot of time any where. You could also jump in and see the Churchill War rooms, the war rooms are not too large and you don't have to spend hours there. All those places are a good walk around. The museums are so large and by the time you get oriented and get something to eat that isn't much time.

Thanks, all.