If you use my u-boot (https://github.com/mvduin/u-boot) with CONFIG_DUAL_RANK_DDR3 set then it will configure ram to use both ranks, interleaved, hence both will get exercised then regardless of how much memory the kernel is configured to use, and regardless of whether it is an LPAE kernel or not.
The main patch for this is: https://github.com/mvduin/u-boot/commit/06e3cb0f3731763372ca437ac0775e040a7d...
This should suffice to test the memory itself.
Making all 4GB available to the kernel right now still requires an LPAE kernel and telling it about the memory available via kernel parameters (I still need to check how to tell it via DT but classified that as non-urgent): mem=2032M@0x80000000 mem=2048M@0x200000000 or, if you're in an experimental mood: mem=2032M@0x80000000 mem=2064M@0x2FF000000 to get back the 16M stolen from the end of lowmem.
Any issues that arise as the result of this are presumably purely kernel issues.
Matthijs