I've been using the 4D a lot in the last few months - my biggest problem with it is that the AF seems to drop out when shooting at higher frame rates.

I've been using it with the E mount unit with Sigma Art Lenses - the AF does work occasionally when first flicking into higher frame rates, but stops working shortly after. I'm wondering if this is a problem with my unit, or whether it's just a limitation of the camera?

Yes I concur.

At 50fps it's started getting really choppy.

Again this didn't happen with previous firmware.

Hi, there.
Sorry for the late reply.
We will confirm this problem with the relevant team and reply to you asap.
Please don't worry and wait for the update.
Thank you.