The development kit spec rumour had 4k output at 30fps and unless Nintendo actually prevent the use of that mode I see no reason why media streaming apps and some small casual games can't make use of it.

720x576 @ 18 FPS

4K isn't some holy grail. I can play 4K video using a 10 year old laptop. Netflix and Youtube could add 4K support without Nintendo having to lift a finger. Switch is plenty powerful for 4K video.

1080p. Most games will probably be 720p or 900p.

Remember even the Xbox 360 and PS3 have some 1080p games with a rock solid 60 frames per second like Rayman Legends. Can't see any reason why Switch can't have a similar platform game at 4k 30fps without breaking into a sweat we just won't be seeing Fallout 5 or Call of Duty Black Ops 4 on Switch in 4k etc.

Yes its possible but that will also limit media streaming like Netflix etc. It may be  commercially a poor decision to actually cap the resolution to a maximum of 1080p. Mentioning 4k would assist rather than damage Switch chances. I don't think people will be impressed with a 720p tablet so at least mentioning 4k when docked wil help.

The dock is irrelevant. A HDMI cable is all that's needed to 'support 4K'. 

People getting their hopes up about the dock are going to be disappointed, imo. It's nothing more than a charging cradle (3DS) and a HDMI wire. It's just a convenient way to charge and connect to the TV.

The dock isn't going to allow higher resolution or more power, imo. What it is going to do is make it very convenient to have a home console connected to the TV in every room. Just not at the same time.

1080p docked. 720p in portable mode if the 720p screen rumour is true.

But those are max res. I fully expect much lower resolution games on the Switch especially XB1/PS4 ports.

Most games native switch mobile resolution below 1080p, some games real 1080p.