Haswell-EP is supposed to bring DDR4 support. GDDR4 and GDDR5 are based off of DDR3.
The Xbox 360 used a 512MiB block of GDDR3 (based off of DDR2) unified for both the IBM CPU and ATI GPU.
The PS4 uses an AMD APU along with 8GiB of unified GDDR5. It's not a matter of whether or not it is possible for AMD to do it because they have already done so. The reason that they have not introduced it to their desktop lineups is because there's no commercially available DIMMs with GDDR on them.