ولو كان مقيماً في أول الوقت مسافراً في آخره يصلي ركعتين
A resident at the beginning who becomes a traveller by the relevant final portion performs two units.
Reverse the earlier case: the time opens while the person is resident, but the prayer remains unperformed until the person enters the legally recognized state of travel. Under the source's analysis, the shorter form applies.
Comparison:
Travel → residence: four units.
Residence → travel: two units.
The rule turns on the legally recognized change of status before performance, not merely on naming the first place where the time began.