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Hi Hamish I'm not sure how many products you have listed on your website but I am only guessing that it is not 4m of even 400,000. I think the question you should be asking yourself is 'do I really need so many URLs?' If you have 50,000 products in your site then frankly you only need maybe 51000 pages in total (including support pages, brands (maybe), categories and sub-categories.  I am only guessing but I would suggest that the other pages are being created by tags or other attributes and that these elements are creating acres of duplicate and very skinny content. My usual question is - 'so you have 400,000 (never mind 4m) pages in Google? - did you write or generate 400,000  pages of useful, interesting, non-duplicate and shareable content? The answer of course is usually no. Try switching off sets of tags and canonicalizing very similar content and you'll be amazed how it helps rankings! Just a thought Regards Nigel Carousel Projects.