I have one single product in over 6 different sizes that is used by 4 different industries. Each application of the product is very different. If I was to create 4 different domains (Example: D1Help.com, D2Help.com D3Help.com and D4Help.com) and add relevant content to each one that is specific to that industries application would it cause me to loose Pagerank or limit me in any way if each of those domains was linked to a 5th site (D5.com) that was strictly an ecom site? I want to create separate domains so that each can have different keyword combination's and rankings. Also each market vertical is so different combining them just would not work.

I would not recommend having separate domains, but maybe sub-domains would work. The best option would be to keep everything under one domain (maybe different directories).

Best:
help.com/d1
help.com/d2
help.com/store

Sub-Domain:
d1.help.com
d2.help.com
store.help.com

Neither one of these options seems to be applicable. We already have an ecom site (D5.com) with a 3rd party, and could be edited as your first example shows:

help.com/d1
help.com/d2
help.com/store

But this would make no sense to our customers because of their diversity. I am also concerned with user expectations and confidence when seeing urls like this. Also how would that effect how Search Engines view us as well. The diversity might create confusion about what we do and rank us lower. I also do not have the option of doing sub-domains with this host.

We do have our own shared servers and would prefer to have domain names with specific keywords included such as:

fertilizerhelp.com, painthelp.com, cellphonehelp.com (these are examples not actual domain names we are using)

A part of our corporate strategy, we do not want each vertical to know about the others.

If it is different content, I don't see why you couldn't use different domains.

I don't see.

If each vertical sector is different, then none of your domains will include duplicate content, which would be where you would get problems with the search engines.

The fact that they are all linking to a single ecommerce site is surely no different than for sites pointing to Ebay stores or for affiliates directing visitors to a single store for that product?

I would go with single brand site and may create sub dirs. or sub domains for other main products pages.