What is the Best way to merge 4 established websites to one Website for boosting brand value?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Kumar Neeraj Kayastha: Best way to merge 4 established websites to one Website for boosting brand value?
  There are 5 websites A, B, C, D and E. A is the company website and other 4 is various services websites. Management has decided to merge all services websites to Company website. All websites are ranking well for different keywords.
  
  How to plan this consolidation that the Company website gain from the other sites links and authority?
  
  1. Do page to page redirect is the best way?
  
  2. Or should I redirect all not found pages to homepage of Company website?
  
  3. Or should I create 4 not found pages for old websites and then guide visitors to the relevent pages.
  
  Or you will suggest something different?
  
  Last Dumb Question of 2014, hope to be on expert panel in 2015...
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  Toni Anicic: 1. Yes.
  2. No.
  3. No.
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  Edwin Jonk: not found pages to homepage of Company website
  
  That is considered a soft 404, meaning that Google will thread it as a 404 at some time.
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  Kumar Neeraj Kayastha: Now It's complicated +Edwin Jonk​​ and +Toni Anicic​​ .
  
  The first action is complex page to page redirection.
  
  To homepage is the easy one.
  
  Somewhere i have read that showing 404 error page is ethically good and vistor go to his choice without confusion is best way. Anybody for the option 3!
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  Toni Anicic: If you're merging those sites into one, we assume you will migrate the content as well. In that case, you don't want to show 404 since the page is not missing, it's just permanently relocated which means it needs a 301 redirect.
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  Edwin Jonk: Part II with +Alistair Lattimore https://www.youtube.com/watch?v=EE6VulTzw00#t=2792