The differences will most likely be in the software, RPi 5 hardware requires 'bookworm' based Raspbian OS.
The first step would likely be to load 'bookworm' on an RPi 4 to identify any software/library/etc issues before the RPi 5 hardware becomes available next week (10/23/2023)...
As for the hardware and benefits for PiDP-11, it is likely 'massive overkill' but in a little while the RPi 4 hardware may not be easily sourced in the retail outlets...
The RPi 5 has higher power requirements (5.2v, 5A), thermal issues (benefits from active cooling, to avoid throttling), and has some nice on-board features like an RTC clock, a power switch connection, and a PCIe 2.0 1x connection for an NVMe SSD, along with a faster MicroSD interface, along with 2-3x increased performance (based on early reports).
A mixed bag of benefits and increased requirements.
Ken
I am looking to buy the new Pi 5 which is scheduled to come out later this month. I am hoping the Pi5 will work 'out of the box' in the PiDP-11. I am planning to repurpose my Pi 4 to control my telescope and try my hand at astrophotography.