Move from 4-day to 5-day week?

[BlueRaspberry7]

I started a four-day week role 5 years ago, on a a pro-rata salary. Since then, the role has grown covering more clients and more deliverables. I've been promoted to a senior role and given a couple of pay rises during that time.

The workload is now a struggle to fit into four-days, and I'm now available full-time. Can I ask to extend to a five-day week and move to the full-time salary, or is that highly unlikely to be accepted?

Yes. It is quite usual for an employee to change their contracted hours, up or down, and for a variety of reasons. If your role has developed and is now more dynamic, then gather the evidence and ask. Follow up in writing via email. Good luck!