Wat do u mean by best distro.
If u want to work with best destro from a user point of view, you may try with Ubuntu. They are going to release a new stable version on 18th of this month.
If you are asking for development, you can prepare any one.
Wait.. U just continue to read my further words..
Me also bought the same laptop, last month. After that I tried to install lot of distros. 4520's has lot of latest hardwares. No one latest distro detected the sound card automatically. I tried Ubuntu edge, feisty, fedora 6 and 7, Open suse 10.2 and 10.3, Debian cant find the video card first of all and some livd cds. Finally I installed Ubuntu gutsy beta version and get a latest alsa source, compiled and installed in the same Gutsy ubuntu. Now its working fine. I need to test wireless and Memory card reader. You may face lot of pblm to find your hardwares, suppose if you will go to old version of distros.