I'm sorry feelingdoneover I am struggling to muster up huge amounts of sympathy for you at the moment. I'm afraid comments like 'thinking it would be a question of looking at my skills and working together to agree a role that would fit' irritate me slightly, and you sound a bit peeved that you can't just say you are coming back 4 days a week as well..... Maybe it's the way you've phrased it but it sounds as though you are cross because they haven't created a role for you which is a promotion on a permanent basis, at the salary you want, to fit your skills and to fit your requirement to work 4 days a week.
Apologies if I am misunderstanding but it does come across a bit like that unfortunately.
I am assuming your acting up into another role before your maternity leave was just that, a temporary arrangement to cover sickness or pending recruitment or similar, with you reverting to your permanent post? You are entitled to return after maternity leave on the same terms and conditions as you would have been on if you were not absent, so I am assuming you are not querying the fact that they are expecting you to apply if you want a promotion from your permanent post?
Assuming that is the case, I think your employers are being entirely reasonable. You can go back to your permanent role no problem. But, anticipating that you will see this as a step back, your employers have identified two other roles that you can opt to apply for. Presumably as you have acting up experience at that level you should be in a good position to apply for one or both roles?
I do think you are a bit unreasonable to feel quite so angry. Have a think about the two roles that have been identified, you could apply for one, then put in a flexible working request to reduce the hours, if you can come up with tasks that don't need to be done, different ways of working more efficiently, etc, to justify a reduction in hours. Or you may have to compromise on your 4 days a week requirement, which is not as easy to accommodate as a job share or similar.
I'm sorry if that's not helpful, I'm not sure really whether you were looking for people to say 'ooh that's outrageous' or honest opinions, but there's mine for what it's worth.