I understand that the Ditto Looper has 5 minutes max of recording time.
Does this mean that an overdub can be erased even if the the initial loop was near to 5 minutes in length? i.e. the initial loop is say 4 mins long and a subsequent overdub loop of 4 mins is recorded. I know some other loop pedals don't support overdub erase if initial and overdub loops total more than the total recording time (i.e. 5mins in the case of the Ditto looper)
No the initial loop will not be erased because the Ditto Looper has double buffer so that it can always store a complete copy of the audio buffer.
Of course, the first loop determines the length of the following loops. This is normal because otherwise your second loop could potentially record over onto the silence from the end of the first loop.