(That way you can prepare image that boots both on RPi and Tinkerboard)
Example hybrid image for Tinkerboard and RPi is here (based on Ubuntu): https://files.husarion.com/husarion-core2-ros.img.xz (unfortunately the build scripts are not public yet)
4 gigabytes of LPDDR3. USB 3. GbE. eMMC. Onboard SPI flash. Unfortunately Mali GPU, but lima is actively developed now…
The biggest downside for me is Rock64 doesn't have an OS with the pedigree of Raspbian. I'm running the 'ayufan' minimal ubuntu xenial, and it seems solid. but when I've played around with a monitor in the full xenial build I've seen some crashes and lockups.
and really any Linux distro can be installed like that
+ "soon", FreeBSD (possible to boot already, but very few drivers have been already written https://lists.freebsd.org/pipermail/freebsd-arm/2018-Februar...)
Not to say Broadcom couldn't spin a newer revision of the SoC with a different internal architecture, but for now it seems that >1 GB of RAM would require substantially more effort than just swapping the DRAM module on the board.
Later versions (I think starting with RPi 2) have a separate DRAM chip on the bottom side of the PCB.