Which should I run first on a fresh install?

its all explained http://www.somethingfornobody.com/blog/morrowind-modding-guide/

The exe optimizer first and the 4GB patch after.

logic:

the 4GB patch applies to any program.

The exe optimizer applies to morrowind.

exe optimizer changes morrowind to a new morrowind, 4GB patch patches any program, including a new morrowind.

have a nice day.

Just install MCP first if you are planning to use it..

EDIT: Praedator's link to Knot's guide explains it well