OVERVIEW:
If you don't need image stabilization (which you don't need as much when it's mounted to the top of your bigger camera) the HERO4 Silver is a really good deal ($40 used). I don't know why footage from this old and inexpensive camera looks so good. I think it could be due to the lack of excessive NR (Noise Reduction) that some of the newer models I've tested have (especially the HERO7). Also, sometimes you might want a bit of camera movement (I.C.M) and for those times this can work really well for that. I keep 2 of these around to use either on the cold shoe of my camera (the "Minimal Kit") or as BTS (behind the scenes) camera, mounted on a small selfie stick/tripod (or a GorillaPod) on the floor. Keep in mind that this one is not waterproof (at all) without the enclosure, so try to buy one with that included, etc.

MAIN POINTS:
PART 1: IS IT PORTABLE?
PART 2: IS IT AFFORDABLE?
PART 3: IS IT COMMON?
PART 4: HOW’S THE QUALITY?
PART 5: IS IT EASY TO USE?
PART 6: NEEDS ADD-ONS?
PART 7: WHERE DO I BUY?

PART 1: IS IT PORTABLE?
Yes, it is super tiny. It would not only fit in a small pants pocket but two or three of them would! I usually store one of them, with a cold shoe adapter, in every camera bag I have (even in the mini one of my "Minimal Kit").

PART 2: IS IT AFFORDABLE?
Yes. On the used market it's around $40 US, and it's so old you won't find many on the NEW market, and if you do they'll probably be overpriced (for collectors to buy).

PART 3: IS IT COMMON?
Yes, it is pretty common (there are lots of them out there) which likely indicates it sold well when it was the current model, probably because it just works good, and it was a big step up from the previous version.

PART 4: HOW’S THE QUALITY?
I have had people comment on how good the quality was (when the footage was edited alongside the Panasonic GX85) so I think it's pretty good. I think I only use it in 2.7K mode (I think that's all that's available in the "Linear" lens angle mode) but it does a good job.

PART 5: IS IT EASY TO USE?
I think so, if all you do is press record. The custom settings are kind of limited though, so you'll have to spend some time finishing the image in post-production. Make sure to use my CUSTOM SETTINGS to get the image to a good starting point though as you don't want to start with an image that crushes the blacks (or blows out highlights) so check out the custom settings for it (on my website, shameless plug).

PART 6: NEEDS ADD-ONS?
Yes. You need to get the waterproof housing, not only for the waterproofing but also for the mounting posts. There are 3rd party enclosures/housings available for it, but because it is kind of an optic as it goes over the lens, I would trust the OEM GoPro version to be less destructive optically to the image...and my custom settings are based on that (it does affects the image).

PART 7: WHERE DO I BUY?
I've purchased two of them o
n local marketplaces (Offer up, Craigslist) because the price has been good.If you try to find them on Amazon, they are likely to be overpriced, but eBay would probably be a good option.

CONCLUSION:
If you don't have a #2 camera, and you have an extra $40, I think the GoPro HERO4 Silver is a good camera to get (do NOT get the HERO4 Black though, as it only shoots in fisheye modes, not in the Linear mode). Even if you do shoot with two cameras already, it can be fun to add some behind-the-scenes (BTS) footage with one of these. Keep in mind that some of these action cameras will stop recording after about 10 minutes, so it's not a replacement for a REAL 2nd camera.