Is there some software that will enable me to search a Word file of
between 70,000 to 100,000 words for up to four different words and
display, in a separate on-screen Word file, every instance where 1, 2,
3 or all 4 (user chooses)of those words appear within X lines (again
user defines) of each other?

This may help, though I don't think it meets your criteria exactly:

http://www.concordancesoftware.co.uk/

There's quite a lot of stuff available, as you will find if you do a
Gurgle Search for "text retrieval".

I've not used any of recent stuff.

Good Luck!