Only one was taken to prototype: the jz4775 because it is fully FSF-endorseable. The jz4760 and jz4770 have Vivante GPU hardware that has traditionally required proprietary drivers, although reverse-engineered Free Software driver initiatives exist. The jz4770 is used in various handheld gaming devices including the GCW-Zero.
Meanwhile, the jz4720 (a derivative of the jz4740, as many of these SoCs appear to be) is used in the Ben NanoNote and is actively supported in the official Linux kernel. The jz4780 is used in the MIPS Creator CI20, but it contains PowerVR technology which is notoriously proprietary and difficult to support with Free Software.
See ftp://ftp.ingenic.com/SOC/ for SoC documentation. (Note that Ingenic's FTP site has changed layout in the past and may do so again.)
See https://github.com/laanwj/etna_viv/ for more information about Free Software drivers and the different Vivante products.