Welcome to the Q&A Forum

It could definitely be a response from Panda/Penguin, as Irving pointed out the best thing for you to do is use a backlinks tool such as Open Site Explorer and take a look at who is linking to your website. Are these websites spammy or could they be blacklisted by Google?

Chas gave a very good answer. And, more people then not will tell you working on 4 similar (but different) sites isn't the best use of time. I am one who understands that in certain cases it does make since to have 2 or maybe 3 sites in the same industry, but I wouldn't go as far as creating more then 1 e-commerce sites dealing with the same products even though the demographics maybe a little different. That doesn't make good sense to me. Your original question I think was more about the server then this sort of advise so I'll give you the best information I have at this time. The sites will be treated as different sites, look at shop services like prostores or stores.ebay they consistently have many people on the same server and all of them are able to be indexed provided they do a fair job at SEO. The issues you'll have is more with building unique and differentiating content for each site and getting good quality back links to each one, then it is with how search engines will treat them. I hope that answers your questions and helps with your goals.

I use WordTracker (You can signup for a free trial) - it lets you type in your domain, competitor domains, and viola - shows you who is linking to where - where you could possibly get links etc. Very helpful tool. I think it would work well in your case because SOMEONE is linking to "boring" content - and having this sort of visibility allows you to see where the story makers are coming from.