There are also differences in some functions depending on having the C revision or not. Seems C revisions are all they are shipping now, but that is somewhat recent and there is nothing in the part number to indicate this.
C version has better PLL, that's what I've been told by Gowin FAE.
There was something else having to do with the BRAMs or the DSP. I checked the FAE email and he noted "We DO support Dual Port BSRAM with Revision C silicon of the GW1N-9/GW1NR-9/GW1NS-2/GW1NS-4 series" as well as saying C has faster speed grades. No mention of the PLL.
Still, what bugs me is the limited info on what the rev C features and lack of markings. At least they assure they are only shipping the rev C at this time. The trick is you need to know to select rev C in the tools to support the new features.
Also, I don't get the lack of LUT RAM in most devices. The -4 is shipping as rev C but the data sheet does not indicate it includes LUT RAM. Their info is not all that good, maybe it's an oversight in the docs. I don't see LUT RAM available for the -4 devices in the rev 1.9.7 tools, so they just don't have it in any device other than the -9. Fortunately the -9 is still very affordable. You don't save much downsizing to a -4 or even a -1 unless you are using many, many units, it won't matter much to use the -9.