We think it will work to skip versions, but as said, specific parings have not been tested so there may be issues we do not know of. I would suggest you make a backup of everything and give it a try. If it fails then you fall back to the step-by-step upgrade.
You do not have to install 1.4.4 as that would be the same as installing 1.4.4.3.
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What do you mean you cannot download 1.4.4?
https://github.com/zenphoto/zenphoto/tree/Zenphoto-1.4.4 (click on the "ZIP" above).
That you can't test all is clear... over all you du a fantastic job here :-)
You do not have to install 1.4.4 as that would be the same as installing 1.4.4.3.