It really depends on whether they're using a single APU or a
CPU/APU + discrete GPU.
If there is just a big APU and a single bus they might go for 4GB of total GDDR5 memory, it would be kinda expensive as it would require to have 16 memory chips on the motherboard (but still nothing like the 60GB PS3 motherboard which had the whole PS2 hardware in a corner
).
In that case they might have to take the plunge at launch considering that less than 4GB of total memory is out of question, then as soon as 4Gbit chips are available they can use just 8 chips, redesign the system and cut their costs as usually.
If they have a discrete GPU then it's much easier.
They can use a split memory architecture just like the PS3, 4GB of DDR3 memory for CPU and 2GB of GDDR5 for the GPU.
Imo this would be a better soution but it depends on the architecture they have chosen, if they're using embedded memory, how much,etc