I'd be a little cautious about doing this in the current Google climate.
One way of looking at it is that Google is going to evaluate each 301 you make - so each URL needs to correspond very closely with the content at the new location. The closer that match, the fewer problems you're likely to encounter.
The other factor is that redirecting multiple domains at once is also not without risk, because it corresponds with a 'buy domains to rank' strategy that Google is (perhaps understandably) suspicious of, or a symptom of a 'failed microsite' strategy where domains are created purely for link building and extra ranking opportunities.
I have made similar moves in the past without any undue headaches, but my opinion is that you should be more careful than might have been the case in the past.
It's absolutely critical that you get the redirects technically correct and the content corresponds as closely as possible.
All that Andy said.... An additional question to consider is whether these "multiple specialty sites" are Exact Match Domains for specific products (to some degree implicit in Andy's comments, but worth emphasizing).
Promoting and then redirecting a keyword-laden domains has probably always been more suspect than the general 301... more so now as Google is devaluing EMDs it feels are lower quality. If these are EMDs, how have they been doing in the current update?
Also, do they pull in links from separate neighborhoods, or is there a bunch of overlap?
1. When you combine A, B and C to D you got to make sure that the content structure remains the same on D as it was on A B and C. I'm not sure how helpful this can be, but it makes it easier for you to manage each redirect.
The last thing you want to do is do a *.* redirect to C. It has to be as clear and distinct as possible or even thinking from a user's perspective, it would be confusing.
Bottom line is you got to make sure that someone coming into A, B or C following a search phrase to landingpage1 from search engines should lead to landingpage1 on either A, B or C or D.
If I were you, my biggest concern would be to make sure that the user experience is maintained in tact.