Photoshop 4 won't run on modern systems, and Photoshop 6 isn't a lot better in that regard.
Perhaps you mean Elements 4 and 6?
I did mean Photoshop 6.0 which had actually been running perfecting well on a 64-bit Win 7 HP computer with adequate memory.. When I had to re-install windows entirely a year ago (HP blundered in configuring the new computer), and now again, I cannot install Photoshop 6, not because of compatibility issues but because it is an upgrade and does not find an earlier version. A year ago, Adobe was offering better (I remember it as being phone) support for Photoshop which it no longer supports except through chat.
Again, Photoshop 6 (OLD) or Elements 6 (only 7 years old) ?
As my first sentence said in the earlier reply, it is Photoshop 6.0 -- I have the disk in front of me and I have re-installed it successfully using using the file and instructions I was given / directed to a year ago when I had to re-install windows. It is up and running fine but it may be the second install allowed under licence as I never installed it on a second machine.
While I cannot be certain of the date, I know I purchased Photoshop 6.0.1 directly from Adobe and I had the choice of it or Elements at the time of purchase. I doubt it was more than 6-7 years ago.