I found something interesting, however this setup is used on Dimension 4700, which is looks like identical to 4600 in terms of case and structural build, motherboard model is different, but motherboard retention bracket looks to be the same, so:
It is very important to get 3 pipe tower, not 2 or less, surprisingly the same part number w4254 corresponds to different looking coolers, so the one I show you below is the strongest, most efficient one.
http://www.amazon.com/Dell-Performance-Heatsink-Dimension-Optiplex/dp/B001PQ5GM0 they have used and new, used $6.99, hopefully not bent, refurbished $30.40
http://www.ascendtech.us/dell-optiplex-gx280-cpu-heatsink-w4254_i_cfncpudellw4254.aspx looks like new or refurbished $19.99 plus free shipping
http://www.impactcomputers.com/w4254.html# refirbished $18.50 I can not see shipping cost
CPU cooler tower w4254, no fan
Fan shroud, this must be purchased or you will burn your motherboard
http://www.centrix-intl.com/details.asp?productid=1076 ship from California, unit price $5.5 plus $8.60 for shipping equal $14.10.
Those 2 parts were tested on Dimension 4700, which looks the same as yours Dimension 4600, I could not find confirmation for your PC, however I did not see anyone upgrading cooling in Dimension 4600 as well.
Is your cooler aluminum, white?
I seen another part F3865 which is belong to your Dimension 4700
http://www.amazon.com/Dell-F3865-H0632-Dimension-Compatible/dp/B00914JP0M/ref=sr_1_1?ie=UTF8&qid=1368062149&sr=8-1&keywords=F3865
http://www.impactcomputers.com/f3865.html?gclid=CPKL1PWfh7cCFYN7QgodSDkAPA#
http://www.centrix-intl.com/details.asp?productid=808
It could be a little inferior to the tower, I shown you first, but in this case you don't need green shroud with fan, you can keep yours.
Also, you
might need retention bracket
http://compare.ebay.com/like/280246343073?var=lv<yp=AllFixedPriceItemTypes&var=sbar
Any questions?