Apple Computer Co. has begun shipping an enhanced version of its Macintosh personal computer with four times the memory capacity of the original, the company announced yesterday.

The new machine, nicknamed the "Fat Mac," represents part of Apple's continuing effort to present the Macintosh as "a serious business machine," said Michael Murray, the Macintosh marketing manager.

The new Macintosh can store more than 512,000 characters, or 512K, of basic memory, thus enabling the machine to process and store large documents and electronic spreadsheets that the 128K machine was incapable of handling. For example, the Macintosh version of the popular Lotus 1-2-3 program will require at least 512K of memory, Apple says.

The new machine is being shipped four months ahead of schedule because of a larger-than-expected supply of 256K random access memory chips. These new 256K memory chips, which went into large-scale production late last year, will succeed the 64K memory chips that currently dominate the market.

Apple also cut the price of its basic Macintosh machine from $2,495 to $2,195, although main computer stores already are selling the machine for $1,995. The Fat Mac's retail price is $3,195.

Owners of the basic Macintosh can upgrade their machine to the 512K version for an additional $995.

Meanwhile, IBM Corp. said yesterday that it has introduced a graphics display and an accompanying "controller" for its widely used personal computer that will enable the machines to perform computer-aided design and engineering functions. The display and controller could be linked to turn the personal computer into "process controllers" that can control scientific instruments in laboratories and factories. The display costs $1,295, and the controller is priced at $2,295.

IBM also dropped the prices on some larger computers and added two models to its line of midsize business computers.