@koken Right, the publisher's first priority should be securing the license for the original VO for the games set in Japan, or in feudal Japan at least, yet here we are: Tenchu, Way of the Samurai, Onimusha and so on. There's few audio overrides in the main game archive, which could be counted on fingers, thankfully (though I still should've properly counted all of the instances), it bugs me a little and I want to ask for help with the archive eventually, but I already have some request placed for the other thing, so maybe some other time.
You'll obtain skins by beating the game, search for the guides on that if you want to know conditions for the exact skins and not bothered by some minor spoilers; you'll also get the generic enemies' skins by carrying those enemies to the base.
There's already one Tenchu 4 undub available, which I played though back in 2014 and recall it being perfectly adequate. Honestly, I haven't touched on the Time of the Assassins until now and just assumed it lacked the voice acting altogether, but thanks for suggestion – I'll take a look at it, since it seems like it would not require much effort.
Tenchu 4 undub is here (may be not the Zairoud's undub I've played):
Time of the Assassins: