My De-10 uses Antonio Villena's 128MB SDRAM "4C", found here:
https://www.antoniovillena.es/store/pro ... am-module/
I want to go to a Dual-RAM setup. Is it safe to add a RAM module from another vendor (such as the following link) or do I need to use another "4C" module?
As far as I know, they're not linked in any way, so they don't need to be identical. They're like totally separate RAM banks on a PC.... within a bank, you want to keep RAM modules the same, but you can have different sizes and speeds in different banks if you want to.
In effect, it's two separate RAM buses, so they just have to react fast enough to keep the core happy, they don't have to be exactly the same.
edit: also, because of soldering differences and the hacky nature of driving DRAM over a GPIO bus, even two modules from the same vendor will probably not react identically. Everyone ends up with slightly different timings. As long as it's reliable at 130MHz, it will work. Most boards go higher; mine does 146MHz for days at a time, and will usually do 148MHz for several hours before throwing an error.
The 4 chip rams do not work as well as the official 2 chip design for the saturn. If you buy ram I would get a 2 chip and then swap that to the primary position and relegate the 4 chip to the secondary slot.