No lucky letters, I think.
But 'Z' is really welcome here in China, because "ZhongGuo" is China in Chinese PinYin.
And Chinese like "Middle" very much, "Zhong" means "middle" in Chinese PinYin. Many many Chinse company name has "Zhong"...
The other most popular letter might be X, Y, S, C, ...
We Chinese like to divide the 4L into two parts, the first two letters and the last two letters.
If the first two is Big city of China, it will be very welcome, Such as: BJ(BeiJing), SH(ShangHai), GD(GuangDong)...
If the last two letters is some kind of business, such as: LC(LiCai, finance), ZX(online), KJ(Tech), SJ(Data), which would be very welcome.
And also "CN" is very popular in China so my ID is DotCN