I am visiting L.A. for 4 days and I am overloaded with choices of activities. What are some affordable things to do in such a short time.
Also, what are the main points of interest that I should place on my itenirary?

Don't know what you like to do! But I would suggest at least one day in Santa Monica--see the pier and spend some time on the beach, maybe walk down to Venice boardwalk. I would want to return to the Getty Museum. Then it depends on what you're interested in.

You can walk among the LaBrea tarpits for free, if you are interested in that. Especially interesting if they happen to be excavating 1 of the pits & you can watch them do that. Then there is a small fee to go through the museum. (This is not dinosaur stuff, but mostly pre-historic mammals.)

Depending on your interests, the following are free or reasonable:

Getty Center
La Brea Tar Pits & Page Museum
Petersen Automotive Museum
Museum of TV and Radio
Hollywood & Highland Mall and the Walk of Fame
Rodeo Drive
L.A. Zoo and Griffith Park Observatory (a must at sunset)
Zuma Beach or Venice Beach
The Subway(MTA) from Downtown L.A. to Universal CityWalk(people watching) or $$$Universal Tour.

Griffith Park Observatory is closed for renovations. The rest of the suggestions are good.