I went with 80 mm VDO gauges and even with small diameter steering whhel managed to get arrangement that allowed me to see all instruments. I think it wil be very difficult to view speedo and tacho if using 4" or larger let alone the other gauges.
I put priority on Tacho then
oil pressure and water temp but managed (just) to be able to see all gauges.
You could always just run 80 mm speedo for engineering. Alternatively see what your engineer will allow.
Regards,
Phil.