Hmm, dunno - it sounds more likely they weren't confident of being able to get 8GB of DDR5 so went for a safer plan. If PS4 had have ended up limited to 2 or 4GB of DDR all other things may have balanced out.
Sounds like Sony got lucky which in business is sometimes all you need
They could have got it but pricing was uncertain. It's GDDR5, which is specialist high bandwidth graphics memory. MS estimated they could put 8GB of very fast DDR3 in (with SRAM buffer) for the same cost of 4GB GDDR5, and that the extra capacity would be of more benefit long term.
One of the reasons that the PS3 remained so expensive was the cost of the memory - some later development of RAMBUS if memory serves. Sony bet wrong that time, got lucky this time.