SEO advice for 4 sites in same niche

Hi all,

I have 4 websites, all on same topic in same niche, but all unique content, with different domains etc.

My question is what is the best way to SEO these in terms of keywords and adwords.

I currently run adwords campaigns for each site - but some work better than others and one of the sites in particular doesn't seem to get much traffic at all.
Should i cut say 2x of the sites off adwords and take that spend and put it in to campaigns for the other two better performing sites?

As for keywords, i currently use mostly the same keywords on each website. My theory when i started was that if i could get 4 sites on page 1 of google that would be good.
But now i'm thinking whether i would be better to change the keywords i use so they are different for each site and aim to get 1x site in the top 3 results for a keyword, then my 2nd site in top 3 for a different keyword and so on.

Any advice on this?

Also, whilst i'm asking - as they are all related sites would placing keyword rich links between the site benefit me?
I read somewhere that google doesn't give links that much weight if they're from sites on the same server - is that right?