According to a Federal Communications Commission filing, there appears to be a new PlayStation 3 on the horizon. The model numbers are CECH-4000A and CECH-4000B; the current model number is CECH-3000. Thus, this new model would usher in the 4000 seres.
The FCC label position on the CECH-4000 PS3 is different. While the filing does not specifically mention dimensions, there have been rumours (via Eurogamer) of a thinner machine.
During the end of the PS2's lifespan, Sony released a super compact (and cheaper) PlayStation 2. While the PS3 already got a thinner version, it's certainly seems plausible that Sony would release a super slim.