The ZXC4 introduces 4 functional improvements over the ZXC3:
An increased FLASH ROM capacity from the 128K of the ZXC3 to a whopping 4096K (i.e. 4MB) - a 32 fold increase! The ZXC4 effectively consists of 256 banks of 16K compared to the ZXC3’s 8 banks of 16K.
The ZXC4 can program all 16384 bytes per bank. The ZXC3 could only program 16320 bytes out of every 16384 byte bank.
Support for the Interface 1 paging mechanism (interception of error handler and CLOSE command handler routines). This allows shadow ROMs to be created to extend Spectrum BASIC, and opens the possibility to use the 4MB FLASH ROM as a Microdrive emulator.
Interception of the LOAD and SAVE routines in the Spectrum ROM, opening the possibility to use the 4MB FLASH ROM as a Cassette Recorder emulator.
The 4MB FLASH ROM of the ZXC4 can be programmed, erased and re-programmed directly from a Spectrum, and so there is no need for an EPROM programmer. The cartridge extends the programmable software bank paging ROM Cartridge mechanism by allowing software to dynamically change the address range used for memory mapped control of the ROM cartridge hardware. At power on the ZXC4 will intercept accesses to the top 64 address locations just as the ZXC3 does ($3FC0-$3FFF), but it can be switched to intercept accesses to the lower 8192 address locations ($0000-$1FFF) instead. Thus when the upper control area is selected it is possible to program bytes $0000-$3FBF, and when the lower control area is selected it is possible to program bytes $2000-$3FFF.
The ZX ROM Cartridge Creator utility (for Microsoft Windows) allows cassette games in PC emulator snapshot files (.Z80, .SNA, .SZX) to be converted into ROM cartridges, as well as binary files (.ROM, .BIN). The resultant compilation can be transmitted via RS232 to the Spectrum at a baud rate of 57600, and the Spectrum then programs it into a FLASH ROM.