RPi4 reboots when playing 1080p content

Hi all.

Ive recently got a new RPi4 2G version, replacing an old RPi2 running OpenELEC which died.

Any time I play 1080p content, it takes ages to buffer, plays the first few frames, freezes up and eventually reboots.

Content is being streamed from a NAS via SMB, using 1gbe -- no wireless or anything in play.

Another Rp3+ plays the same content without issue, along with Kodi running on a Xiaomi Mibox S - identical configuration etc.

Is this a known thing with the RP4 ?

TIA!