I am planning different options for a 4 day trip. I got cheap flights for a place called Clark in Philippines from Singapore. But I don't know much about Clark and Philippines. I want to know what are the places I can visit If I go to Clark.?
Clark itself is not worth visiting. Until 1991 the area used to be an American air force base: 'Clark Air Base', now its an airport which some budget airlines use. The area is also a free trading zone: Clark Freeport Zone, but not really interesting for tourists, unless you are into old US military installations.
The main reason to fly to Clark, is that is usually cheaper than flying to the main Manila airport, but you don't stay there, you move on to Manila (60km south) or to the Northern parts of Luzon.
There are direct buses from outside the terminal to Manila, but you can also walk to the main road take a jeepney to the next town and then a regular bus to Manila, the second option is cheaper but will take longer.
Manila is a big city with some sites, but you may not need 4 days unless you want to go shopping which should be cheaper than in Singapore.
Alternatively you could head north, there are some awesome rice terraces around Banaue, Batad and Bontac. There is a lot of great nature in the hills of Northern Lazon.
Clark US Air force Base was used a lot at the time of the Vietnam war. It was a stop off for American GI's debrifing and transition solders back into the world as we new it then. It has nothing much to do there, so don't expect much to do if you go there.