Why were only four carriage services running between Guildford and Surbtion on Tuesday? I assume this is still the case. There are carriages in Guildford sidings as my photo below shows or would do if I could add it here. It keeps failing to post.
Because SWR are charged lower track access and traction current costs by Network Rail than for an 8, 10 or 12 car train.
It may also be specified by the contact with the DfT.
The third alternative is that 4 cars had to be detached as a failure and the ability to attach an extra unit depends on having a reasonable ( 10 minute) turnround and a spare driver to shunt the unit to be attached.
This is not an official answer - just one from a former railwayman....