In my experience it very much depends who you work for. If you work for a small private company it is unlikely they will have set rules. Unfortunately there ares no Job Centre or DWP recommendations.
I phased back after 9 months off into the NHS. My GP said Phased return, Occupational health said 'very slowly' and my Manager interpreted that very kindly. I did week one 2 hrs per day 2 days, week 2 4 hours per day 3 days etc for 8 weeks then i took holiday Tuesday and Thursday for a few months. From the very first Monday i was paid in full throughout. I also know colleagues who haven't been treated so well.
I think i was lucky and it very much depends on the organisation and who is negotiating on your behalf. Maybe you could speak with Occupational Health or look at the Local Education Authority Policies for Employees, Do you have a Union that could offer some guidance on what other people have been offered.
Take it easy and the best of luck getting back into your routine. Butterfly x
I am no expert but I would think the same as you, it is 4 actual weeks of working that would be your phased return so the week off does not count, I am sure one of the other ladies on here will be able to advise.