I've got a couple ideas here.
1 - Set up Cross Domain GA Tracking
This would measure literal overlap in users over time. Basically - has this person been on any of our sites before?
I'd probably do this with a new GA UA account to keep the current accounts the way they are.
If you're not doing affiliate tracking between the sites then this added analytics layer may be a good idea anyways.
2 - Set up Leadfeeder.com accounts
This can show you some information based upon user IP address. Not much different than GA data but fun to look at.
3 - Analyze email lists
Assuming you've got lists built for each of these sites, compare them in rapleaf or full contact and do a subjective/manual review of the customers that you can pull data on.
4 - Set up retargeting scripts on all of the sites, and then run some retargeting campaigns
You should be able to get some general insights from Facebook, Twitter, etc on basic audience demographics. Basically building a custom audience and then pulling them into Facebook audience insights once you have enough people in that audience group for each site.
5 - Check social insights
If you have any social followers that are separate you should be able to compare page owner insights on these audiences.
6 - Keyword Overlap
Use the domain comparison tool in SEMRush and see how much ranking overlap there is. May or may not be relevant to the audience, but I can make an easy argument for a site merger if there's cannibalization there and neither site is ranking 1st for many of those keywords.
7 - Look at conversion data
If you're getting a strong conversion rate from referral traffic from sites B, C, and D, that should bolster your argument. By strong I mean above your site's average conversion rate.
General takeaways:
From what you've said so far, I think you can make an easy argument to merge sites C and D into the main site.
Site B is the harder decision, especially since I'm not sure how many constraints you have in regards to claims made on site B and the laws surrounding supplements and vitamins. Microsites can go either way IMO - I've seen great ones and I've seen some that never should have been separated. There's an easy SEO argument for merging them but it may not be the best brand decision.
A good relevant example is the bulletproofexec.com site and their upgradedself.com ecommerce store. Pure SEO standpoint would be to merge these 2 sites, but I think they've got a strong brand built up in one and they may not want to sell those products on their main lifestyle/blog site.