They should make you permanent now if they are recruiting you in to the role of the person that isn't coming back. Ask them why they are not making you permanent now?
Is it because you are pregnant? Probably, in which case its sex discrimination!
You need to challenge them on this one as you have a lot to gain now by becoming permanent.
With regards to the 4 days, you have to be employed (on a permanent contract) for 26 weeks before you become eligible to apply for flexible working.
I wouldn't mention it now, if they are not keen, it may affect the offer of a permanent role anyway and you won't have any redress.
Sorry if I sound cynical - not all employers are bad but they do need to make you permanent now.