Parsing and comparing four documents with likely known and unknown differences could explode into quite a lengthy and cumbersome report.
Depends a great deal on what specific differences you are looking for....
First:
Regarding "Paid websites will compare more but presumably they only accept documents in text format ."
Contact those websites and ask. Ask about the process(es) used and costs. Security may be an issue so check on that . How reliable are those sites?
Second:
Without knowing your working environment (and not being critical either) if you are in a situation where four copies need to be compared then you probably need to revise current workflows and/or look for specific software to help manage document workflows and the revision process.
Configuration management may be more applicable.
SharePoint being one immediate possibility since you are already a Microsoft user (Word and Excel).
Details matter and your needs may be more configuration management related versus document/file management.
Third:
Keeping the document management process in house will likely cost less, be more controllable, and more flexible.
There is quite a bit of information on line regarding document management and the associated configuration management.
Hopefully some of that information will be directly applicable in the form of best practices, processes, and available software.
The following link may provide some ideas and possible products.
https://www.softwaretestinghelp.com/top-5-software-configuration-management-tools/
You can easily find similar links.
Some manufacturers will provide the software free for trial purposes. Set up a test environment to do so.