I missed where you said which PW4 model you are hoping to mod, or how much storage you were seeking? You do know that there is a 32GB PW4? If you are still inclined to proceed, then I would think that @knc1 's OTG suggestion would be an order of magnitude easier to implement than replacing the eMMC chip.
SMC BGA hot air rework is not out of reach of an advanced hobbyist, but it's not for the timid either. There are forum posts, blogs & at least one video on the net of people increasing capacity or replacing defective eMMC chips on at least one netbook, tablet & PW2 as well as nexus, samsung and iphone mobiles. The netbook upgrade was well documented, had an accompanying video and IIRC replaced a 32GB eMMC with a 64GB chip. You're not likely to get away with simply replacing the chip. You'll likely have to migrate firmware as well. This forum has an outline of what needed to be
migrated in that case for a PW2.
I read somewhere that the PW4 processor is
iMX6SL. But NiLuJe is a more reliable source. Have a look around NXP's site for a data sheet. OA2/3 documentation
here.