Can someone confirm if the LS300 will shoot:

4K (4096x2160) @ 150Mb when set to 25fps, with the latest firmware.

C4K is still 23.98 or 24fps

The camera has a number 4K settings.

C4K (4096x2160): As Noa said, this limited to 23.98 or 24fps - To match cinema frame rates. This can be either 150 or 70mbps.

4K (3840 × 2160): This can shoot 25 or 30fps depending on your region. It can be either 150 or 70mbps.

I would assume it's best to shoot in 4K 150mbps at 25fps and then if you want the aspect ratio of the C4K crop a bit off the top and bottom. You lose a few pixels, but honestly, no one would ever see that.

That said, if you really want to have that extra width of the Cinema 4K just shoot at 24fps and then conform it later. I've used Cinema Tools to do this, it's very fast and the end result is flawless. The only downside is you also need to convert the footage to ProRes before Cinema Tools will recognise it - That bit takes a while.

The way you chose to process the material is up to you, but there's less room for error to just shoot at 25fps and crop. This would also be the faster workflow.

In a bit of a pickle. Client is from the UK. Shooting an interview (in the US) for what they say is a "theatrical movie" in 4K 25P. Their normal camera is the Canon C300. Not an expert on that camera so not sure if it DOES does C4K at 4096 or not.

Am I stuck?

Well, best to be upfront. Client is fine with 3840, so all is well.

Wonder why JVC stopped short of making 25P mode full 4K cinema specs?

BTW: C300 Mk II does shoot full 4K 4096 at 25P

I mean, they only need to upscale it by like 250 pixels, no one will ever see that. I would have thought matching frame rates are much more important. Glad you got it sorted though, let us know how the shoot goes and how you get on with the camera.

I guess that extra 1fps in C4K is the difference between a $2.5k camera and a $15k camera haha.

A theatrical movie at 25p? Unlikely. Something that will play in a theater that incorporates 25p footage sure. I have this problem all the time with European film shorts made for TV that are 25p when they get mixed with 24f material in the same program. It's easier to bring everything into a 60p final edit for projection than try to reduce 25p to 24p.