4K (UHD) and fullHD does have the same aspect. Most 4K Projectors does scale HD to UHD and from and to any other supported signal seamlessly. Therefore no projection area is lost, nor is brightness. This is only the case if the projectors native aspect doesn't fit to the source aspect (eg. FullDH source to WUXGA Projector 16:10). But 4K (UHD) and fullHD does have the same aspect.
Most 4K Projectors does scale HD to UHD as well from and to any other SUPPORTED signal seamlessly. Therefore no projection area is lost, nor is brightness. This is only the case if the projectors native aspect doesn't fit to the source aspect (eg. FullDH 16:9 source to WUXGA 16:10 Projector).
The specific projectors main Specs say
Resolution 4K (3840 x 2400 pixels) (Quad Pixel Drive: ON)
This is the native chip resolution (* see Pixel shift at the end of this post):
Looking at the detailed specs (page 17/25 and following) you'll find a list of all supported resolutions.
Maybe you could provide the exact issue (screen shots), which results in 'I don´t find a way to change the projector resolution.'?
*Just for the nerds: Only very few of the projectors on the market provide real native 4k Chip resolution but use pixel shifting.
Basically, the RGB pixels aren't projected on the same spot, but are shifted a little bit, in extra frames. An RGB full frame picture this way becomes three frames R +shifted G +shifted B picture, resulting in a higher virtual resolution.