The data sheet seems to be the one for E-350 though.
The G575 series of system contains a variety of sub models, some of which contain 2 DIMM sockets and some which contain only one socket. Each socket can support a 1GB, 2GB, or 4GB module. Models with a single socket can be upgraded to 4GB of memory while those with 2 sockets can support up to 8GB.
Common models of type 4383 with a single socket are -42U and -43U
8GB modules have not been tested by Lenovo and may or may not work. A 64 bit version of the OS is required to access more than 4GB of memory regardless of the physical amount installed.
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Here is a view of the memory compartment of the single dimm equipped model 4383-43U. The highlighted area indicates the solder pads where a second memory socket would be installed on 2 dimm models of the G575.