Changes between each release can be found here
https://www.memtest86.com/whats-new.html
As of Aug 2020, the current release is V8.4.
There shouldn't be any need for V7.x, as V8 works on all the same hardware and should be better in every respect.
However if you are using a really old machine without UEFI style bios, then you might need V4. Which can be found on the MemTest86 download page
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