SOLVED. Apparently [referring to RPi Org doco] when connecting headless and using an ethernet cable between client and RPI4 the password keyed might require the keyboard locale to be English-PC. The @ symbol is down at Shift-" and with that the password was accepted.
ADDITIONAL For completeness of answer.
RPi 4 now working on MAC Book Pro 15" (mid 2015) via CAT6 cable connecting MAC Book Thunderbolt - Ethernet adaptor to RPi Ethernet port. Set at full 1GB/s.
The Mac was using its internal keyboard with the AU setting originally and its internal trackpad/or the attached wireless mouse (HP) and the standard retina display.
With respect to the post topic - regardless of what I saw on the screen being typed, and it looked OK, when it came to the RPi, user Pi, password whenever a sudo or similar root command was used, it required me to have the keyboard locale in British-PC. What I did notice of course is that the @ is Shift ’ on the British keyboard.
That is not all. The ethernet connection was flaky and I could never (command line) get the RPi to ‘fix’ a static IP address. Thats just my lack of skill. ALL the tutorials etc were assuming that there was a ‘network’ of some sort (i.e. network name, group etc) with many of the commands not delivering anything like the examples. So looking in the MAC Network connections or using the terminal really didn’t assist in fixing it. They [variously] did give me the ‘current’ RPi IP address as it kept changing on RPi reboot. Typically I saw something like 169.254.nnn.nn. or even 192.168.0.1 once. I kept trying with the current list until I got a successful ping. The Ethernet connection was never fully green (it was occasionally) but if I could get a ping then I could log in.
Finally I am in and working on the desktop of the RPi4, via a cable between Mac and RPi (not other network, no WIFI or BT). To get this working I needed to:
a. change keyboard input source to British-PC (just for the period to get the VNC working),
b. get ip of RPi. Mac terminal > netstar -nr. Then ping raspberrypi.local or the ip address to confirm active on this session of ethernet. When OK. To ssh firstname.lastname@example.org. Password with UK keyboard required at this point. Then RPi local up. sudo raspy-config to get to configuration menus. Configured the locale, speed, boot up, turn on predictable interface names, ssh on, vnc on, updated the software (note that the MAC was ‘hot spotted’ to my iPhone in order to have internet connection), and then restarted RPi.
c. Apple-G to launch the VNC (built into Big Sur). I got the IP address and when i tried I got a warning that the VNC was not compatible. Researched and I then downloaded a new VNC ([VNC Viewer 6.21.118 Real VNC].
d. subsequently got a warning “VNC cannot currently show the desktop”. Needed RPi to boot to the desktop [selected …and already be logged in option] and that the screen resolution needed to be set as high as possible (from cl sudo raspy-config option).
There are articles about this on the web as far back as 2014 I think. Still no joy.
e. More research and downloaded a new desktop for RPi. LXDE I think - at command line inside the RPi using sudo apt-get install lxsession. Rebooted.
f. Back into the MAC, used the new VNC and pop - right into the RPi desktop.
The desktop launched straight into some configuration items which I followed all the way through including setting up a good password. Changed a RPi password setting to use the VNC Password instead of the Unix one and set a strong password for this. Turned encryption always on so that more than just the first 8 characters of VNC used. Set Pi user password to ‘remember’ so that the long string is not required to log on each time. Also set the new VNC server on the MAC to require a Master password for each ‘session’. Again using a strong password.
So ultimately we are into desktop on reboot & launch of VNC on MacBook Pro. Well twice in a row - not a statistically good number but will take if for now. Still cannot get ‘internet’ to the RPi when at that desktop so that is the next challenge.
In summary; ip, keyboard to get login then sudo config (changed settings and got latest software (4Feb2021)), new vnc on Mac, new desktop on RPi, changed RPi screen resolution to max. Rebooted RPi then restarted MAC and launched VNC. Pinged for latest IP and then VNC in and it worked.
Hope this helps someone.