Qsys with up to 8 GB of DDR2

Although I haven't done this on the Stratix 4 board yet (haven't needed that much memory yet), you can use the AXI bus in QSYS and get up to a 64 bit address space.

I have used AXI with the Cyclone V SOC kit, and with the Xilinx Zync platform, and it's a powerful bus interface. The Problem is this doesn't seem to be supported by NIOS II yet. (Only 32 bit CPU, so it's tough to address > 4 GB)

The best solution I guess is dependent on your real requirements. If you need it for code space you can do blocks of memory, but it's a pain to manage. If you need it for physical data space for the rest of the hardware, you can create a custome interface, that gives avalon access to 2GB, and the hardware the the rest of your FPGA hardware the rest.

Pete