What happens to my job if I take unpaid parental leave of more than 4 weeks on top of adoption leave?

You have the right to return to the same job that you were in before the parental leave period, but if your employer can argue that the job no longer exists or they could not hold it open while you were away then they have to offer you suitable alternative employment. This can be with the employer themselves, or with an associated employer. It does include an employer who took over a company from your original employer. The job must be very similar to your previous job and have similar terms and conditions.