Sam
Indeed, the tiny lenses on iPhones and Androids are detrimental to great quality video especially in low light situations. You are absolutely correct in assuming that the small opening does not allow enough information into the sensor. The smart phones and a lot of the digital cameras these days use software based aperture solutions which utilize what little light is coming in and compensate mathematically to formalize the appropriate imagery. But, just as having a peekaboo pin hole on your front door, only a tiny bit of light will get through and your entryway will still be mostly dark with only shadows and murkiness. But put in a big three foot square window and your entryway is flooded with light and you can easily discern the Picasso on the wall.
So If you want to shoot in low light, like maybe a night scene on your layout. And if you want to see all the appropriate detail in high resolution, you are still going to get far better results with a larger opening that lets in as much light as possible. Software does wonders, but it doesn't stack up to analog light focused through big glass.
Shooting your layout in a non-brightly lit room, even in 4k, will result in muddy viewing. I use my iPhone all the time to shoot my kids on the train at Disneyland (that is the obligatory reference to trains to keep my post from raising the ire of the management). I use my iPhone because it is simple and because I am shooting in bright daylight. I still use my Nikon SLR to shoot videos indoors when the lighting is dimmer and artificial. And I get far better results even though the Nikon is capturing in 1080p (which is a quarter the resolution of 4k).
If you are downloading directly into your computer, I assume you are loading into an editing program? Both Avid Media Composer and Final Cut X can edit natively in 4k, but you need a lot of memory, a fast processor and a Thunderbolt or HDMI cable to your 4k compatible monitor. Also, if you are really into maintaining the native resolution, you also need to be aware of the codecs that you are utilizing for conversion. For instance, many Mac users default to working in the .mp4 format which typically utilizes the H.264 codec. They don't realize that the H.264 solution is not optimized for working with 4k and they have already compromised their video without knowing it. Yeah, yawn. Terribly boring stuff.
Then, when you are done editing natively, you need to output to a native 4k format -- which won't be viewable by most people anyway in full resolution. If you upload to YouTube, you are already compressing the video for transport. And your users on the other end have conversion solutions happening on their end right inside their browsers when they are downloading or viewing your masterpiece.
But that's all mumbo jumbo voodoo technical nonsense that really only matters if you are going to be outputting native 4k in order to present your video at the Smithsonian IMAX theater (but in that case, you should be shooting in 8k...).
Again, 4k doesn't really matter to most mortals... yet. It is optimized to look great on REALLY big screens and also when you are sitting a proper distance from the screen. Otherwise, you aren't going to perceive any difference as a regular human from what you would see on boxing night on HBO in 1080p on your 70" screen.
Basically, without getting all silly about this, if you are making videos to upload to YouTube, keep doing whatever you are doing -- if it looks great to you and the users on the other end -- then it's perfect. You don't need to worry about any of this nonsense. I only know two people who really and truly understand all these acronyms and codecs and formats and pixel depths and aspect ratios. And they are not fun at parties.
Have fun!