More reasonable speculation might be that the new model features a new connection to an optional 16GB chip on the motherboard. Or they don't want to keep the old production line just to build new debug units, say.
I guess that depends on why Sony would go with a new chassis designation from 3000 to 4000 model.
Sony in 2009 went with the 2000 designation with the Slim and a reduction in die size for Cell, RSX, change in the HDMI chip and the addition of CEC support and a change in the outside case along with a release of firmware 3.0.
So what triggered the major change in chassis #. In the 3 years since the Slim launch there have been changes in models and minor changes in model # and a change from 2000 to 3000 with the RSX Shrink but not Cell.
The major
software change that came with, at the same time as the 2000 Slim was Firmware 3.0 and planned was support for 3-D and Sony starting to implement support for IPTV. To this point Sony has not implemented IPTV or HTML5 <video> but has been putting in place the libraries to fully support a webkit2 browser and IPTV, in other words XTV. They transitioned to firmware 4.0 without a new chassis and from past experience Firmware 3.0 supported both fhat and Slim except for CEC commands. So while my opinion is that major Firmware version number changes loosely tracks changes in major model number changes because there is a long range plan in place, it does not dictate the change. (Still think 4K resolution is coming during the 4.X version numbers and it may be supportable on older models. Firmware 4.0 may signal OpenCL support, h.265 as well as Full HTML5, WebGL, WebCL and AR (most of these changes would only apply to the XMB side not game side)).
The amount of change is debatable. My opinion is major change and it's just that, a guess based on the trends I've noticed and the published patents that could support such a major redesign.
So given it's not going to be just a dumb shrink for Cell (everyone agrees that will not be possible) or changes in top loading or slot loading Blu-ray player, what changes are coming?
Sony tried to hide the Slim FCC registration from us by registering the PS3 Slim 3000 model using a Shell corporation with a different name. The press saw through that and leaked the information that a Slim was coming and Jack Tretton mentioned Sony's displeasure that they couldn't keep a secret. Super_secret was also leaking information about firmware 3.0 and a new chassis.
This time they are not trying to hide that a new model is coming but are hiding pictures of the outside and inside as well as owner manual and theory of operation. Hiding that it's a top loader? Smaller? No, something else is coming with this new 4000 chassis.
"a new connection to an optional 16GB chip on the motherboard." Possible but I don't think they would totally redesign the PS3 chassis to just support a cheaper interface to a serial Flash SSD. They can do so with economy of scale and support a SSD drive that is just internally attached to the same SATA interface the hard disks attach. It also gives no advantages unless SATA is upgraded to a newer faster SATA interface. A newer faster SATA interface or USB 3.0 (maybe not fully) is a major major redesign and not possible without more than just replacing a SATA or USB support chip as the data thru-put is massively higher.
AMD PCIe and Southbridge would support this but would require faster memory access as would Flex I/O. Cheaper DDR3 memory could be used with a 256 bit buss not 128 bit. XDR (Rambuss) faster memory was required because of motherboard trace length and a choice to go with 128 bit wide memory. It's a differential pair for each buss line to memory (costly).
We now have TSVs and substrates between Chip and motherboard that can easily/cheaply support SHORT traces to 256 bit wide memory. That is a major game changer....
more expensive motherboard and XDR memory is no longer needed. Cheaper custom 256 bit wide DDR3 memory can be used and it's also fast enough to support USB 3.0 and faster SATA. This same TSV technology inside memory chips is making possible serial stacked Flash ram with logic layer on bottom and stacked custom high density ram.
We already know a PS3 Cell refresh to 32nm is going to require a redesign as XDR and Flex I/O interfaces on each side of the chip can't be downscaled.
Multiple thru Cell connections using TSVs will be needed rather than current connections at each end of the chip. A new memory interface to L2 cache would be needed and with TSVs a larger buss width for cell is possible. Using a large multi layer substrate between SOC chip (CPU, GPU and more on one SOI Silicon chip) and the motherboard. The substrate would have custom 256 bit wide memory attached to it.. Using TSVs in Chip to connect to the substrate to support connections between SOC chip and memory. Short trace length and 256 bit wide memory would allow cheaper DDR3 memory to be used in a PS3 instead of XDR RamBuss memory.
This is only one of a possible number of changes that could come because of what TSVs make possible. My understanding is the Cell design was best design at the time given TSVs and custom memory were not available. Connections internally to Silicon were wire bonds connected to pins and the interface pins had to be accessible at the edge of the chip. This is no longer the case. This opened up new chip design models and now we have CPU-GPU and more on the same silicon. Because of this the PS3 Cell design is dead but not PPUs and SPUs.
IBM as of Dec 2011 is making a all on one SOI wafer @32nm CPU-GPU-eDRAM for WiiU and what else as both GF8 and IBM are both making these chips.