OK. Solved.
0. Samsung Devices
There are font-changing apps for this. Appears to work, I don't have a Samsung device.
1. Other Devices: DON'T use font-changing apps
There are font apps out there that may or may not be updated properly for Android 4.x ~
BRICKING YOUR DEVICE CAN HAPPEN
2. Why bricking happens
(a)Wrong metadata
(b)Wrong permissions
3. How to solve
OK. Firstly this requires root, and you MUST BACKUP with TWRP Recovery, etc. or otherwise say hello to Brick City. You MUST KNOW what you are doing, I won't be held responsible, etc. etc. This is not a guide, just a quick update.
Download the font of your choice that you would like to use. Here's the trick: if the font is "Bold" it can't just say "Bold" in the filename. The ~metadata~ must be Bold as well.
How to check: On Mac OS X, open the font using Font Book (comes with OS X). Choose Preview > Show Font Info. Look under "Style" ~ this must be Bold if it is Bold. Example: Cabin Bold has Style "Bold". Hence you can replace Roboto-Bold.ttf. Walkway Bold, Black etc. has Style "Regular" ~ you can replace Roboto-Regular, but you will brick your device if you replace Roboto-Bold, because Android says, hey, this isn't "Bold"!
For Windows, download TypeLight at
CR8 Software Solutions ... Now I haven't really tried it, but open the font file, then in menu choose Font > Description... Same thing here, Bold must be Bold if you are replacing Roboto-Bold. You can apparently also edit this accordingly in TypeLight but I haven't tried.
Next, assuming your font's metadata is fine, make a BACKUP of existing Roboto-Regular.ttf and Roboto-Bold.ttf using File Explorer or suitable app. Copy the ~new~ font file to /system/fonts, overwriting Roboto-Bold.ttf etc. using the proper name. That is, supercoolfont.ttf needs to be copied to /system/fonts with the name Roboto-Regular.ttf or Roboto-Bold.ttf depending on what you want to replace. If the new font is .TTF, that's fine. If the new font is .OTF then change it to .TTF, this seems to work.
After overwriting, DO NOT reboot yet.
You must set the permissions of the file to RW-R--R-- or bricking can/will happen.
OK, now reboot and everything should work. If it doesn't, please post below.
REMEMBER ~ if you BRICK your device just use TWRP or similar File Manager ~ so just delete the new font and then rename your backup to normal eg. rename Roboto-Bold.ttf.backup to Roboto-Bold.ttf so in essence Android doesn't see any difference and this should unbrick things. You do not need to restore from backup, reflash ROM etc. because it's just the new font file with wrong metadata or wrong permissions that bricks the device.