1.618 = 1618 / 1000 
Both 1618:1000 and 1000:1618 work for me in PL4, but not necessarily with the expected effect. (Although I would personally live with the builtin 16:10 ratio in this case.)
The first time you enter your own ratios the behaviour is an improvement over PL3: the first ratio above gives a landscape crop, the second a portrait crop. Even changing the ratio behaves better than in PL3, with the crops being centered on the image instead of the wonky behaviour I reported here.
Now move the crop, press Reset, and try it again, and the behaviour is different.
So much for consistency, but then they’ve only had 10 months …