I can confirm it works on both XF 1.3 and XF 1.4.
@skhms could you edit the resource to officially list the add-on as 1.3- and 1.4-compatible in the Overview tab? I was actually going to make my own until I found out yours works perfectly well. I think people might be turned off from downloading it because the Overview tab says it just supports XF 1.2.
Also, the add-on's title in the admin panel add-on list is "SK Mark Thread as
Thread" which should be "SK Mark Thread as
Read", just pointing that out.
Excellent add-on! Thanks.