Thank you, raymondcc. For our website, the 404s are all because of -
1. Disqus plugin. I think 70-80% of all 404s are because of this.
2. Deleted tags. I deleted about ~25k tags which were totally useless; but were indexed by Google.
I've redirected the deleted tags to homepage and those generated by Disqus have been redirected to correct URLs.
Could you share the fixes that you applied to your site so that I can do something similar?