ZoneMinder has been around since 2003, and there are bits of information that used to be true for a long time, but are not any longer, due to changes in the software. Since version 1.25, ZoneMinder uses mapped memory by default instead of shared memory. Unfortunately, old habits die hard. Many who write documentation or personal blog entries still include the shmall/shmax settings because that is what they have always done or, like you, they Googled and saw someone who did that. This has reached Urban Legend status.
Yes, try to change your cameras to 32bit color depth. It will not hurt to try. It will either work right away, or it won't work at all.
http://libjpeg-turbo.virtualgl.org/
The libjpeg-turbo library has been expanded to make use of NEON instructions, which your cpu has, so let's see if it works.
The name "32bit color depth" doesn't actually mean what you might think it does in this case. I don't believe it uses more memory than 24.
2fps is perfect for a general surveillance system.
I just want to double check the screenshot, showing the Remote Host path, really is exactly the same for all three of your Foscam cameras.
The snapshot url you are showing (CGIProxy.fcgi?cmd=SnapPicture) applies to older mjpeg-only Foscam cameras. I cannot find any documentation stating that snapshot url will work with your newer h.264 cameras. In any case, what we really need to verify, is you really are grabbing snapshots, from all three of your cameras, and not any kind of a stream. If you are grabbing a stream, from any of those cameras, then you have got to clear out the MAX FPS and Alarm FPS fields or you are guaranteed to have problems. I can't tell you how many people ignore this warning, and then report problems in the user forum.
Your system load is a little high. See how htop shows red bars for all four of your cores? That means your system is barely keeping up, and all it takes is something else to trigger in the background (zmfilter, zmaudit deleting expired events, cronjob, logwatch, etc) and things can start breaking.
The log entries you've provided have all been with cameras 3 & 4. Have cameras 1 & 2 ever stopped working?
When a camera goes offline, look at all your system logs under /var/log. Look for other events going on, during or just prior to, a camera going offline. This can be a little like looking for a needle in a haystack. Try sorting the folder (and subfolders) by time, and see which log files got written to at the same time the camera went offline.
As a second experiment, disable cameras 3 & 4 for two days. If camera's 1 & 2 remain online, reverse the process. Disable 1 & 2, then reenable 3 & 4. Wait 2 days. Report the results. If no cameras go offline, then try running with cameras 1,2, & 3 but leave 4 offline. Report the results.
I know this can be inconvenient. Having a second, temporary ZoneMinder server around would come in handy for this test so you don't loose any video. ZoneMinder runs well in a virtualmachine. You could download one of bbunge's pre-made ZoneMinder images and run it in a Virtual Machine on any Windows laptop or other pc you've got lying around.