In September 2012, Sony announced that a new slimmer PS3 redesign (CECH-4000) was due to be released in late 2012 and that it would be available with either a 250 GB or 500 GB hard drive.
In PAL regions, the 250 GB model is not available; a model with 12 GB of flash memory (CECH-4000A) is available in its place. A standalone 250 GB hard drive (CECH-ZHD1) is available to upgrade this model. In the UK, the 500 GB model was released on September 28, 2012, while the 12 GB model was made available on October 12, 2012.
CECH-4000B consoles (those with hard drives) weigh approximately 2.1 kg (4.6 lb), while the CECH-4000A weighs approximately 2.0 kg (4.4 lb). Both are ≈25% smaller and ≈20% lighter than the original PS3 Slim. This version has a sliding disc cover rather than the slot-loading drive found on previous PlayStation 3 consoles.
Source: Wikipedia, "PlayStation 3", available under the CC-BY-SA License.