denisl - 10:25 pm on Feb 8, 2013 (gmt 0)
I dont think the 404s do any harm.
A site of mine was hacked about 4 years ago. I had a stupid redirect script in a page that was taken advantage of by p#%*n sites though I quickly removed the page and got it removed from G.
The site took a knock but recovered its position in a few months.
However WMT is still showing around 60,000 404s which are links with various paramaters to that non existant redirect page. Although i would like that site to do better than it is, I dont believe those links to 404s are hurting it - even if I worry that they might.