I was there in 2002 and 2012. The first time was during the SARS epidemic so there were not a lot of tourists anywhere in Asia, and most were Westerners. There was a lot more in 2012, with a lot of Chinese and Korean tourists. The biggest change was the addition of wooden stairs for some of the steeper rock stairways. Easier but not as challenging. I recommend touring early in the morning and relaxing in the afternoon. With 4 days, must sees are:

Angkor Wat - The most exotic place I have visited in the world
Angkor Thom - the tall Peminenakus (spelling is wrong) temple is my favorite place if you like climbing temples.
Ta Prohm for the trees
Bantea Srei for the carvings
Phnom Bakheng for the sunset (it will be crowded)

Enjoy but do not wear your self out.