In 1980, the 4000-series PETs were launched. These used a larger 12" monitor with a redesigned CRT controller and also included the enhanced BASIC 4.0, which added commands for disk functions. By this point, Commodore discovered that people were buying cheaper 8 kB and 16 kB models of the 3000-series and upgrading the RAM rather than pay extra for the 32 kB model. Because of this, they punched out the memory sockets in the 4016 (there was no 8 kB 4000-series PET) to prevent that practice. The 4032 was a huge success in schools, where its tough all-metal construction and all-in-one design made it better able to stand up to the rigors of classroom use. Just as important in this role was the PET's otherwise underutilized IEEE 488 port. Used wisely, the port could be used as a simple local area network and allowed printers and disk drives (which were then very expensive) to be shared among all of the machines in the classroom. Unlike later Commodore machines, PETs had no kernel ROM functions for the IEEE-488 port and users had to write their own for using peripherals such as modems.
Source: Wikipedia, "Commodore PET", available under the CC-BY-SA License.