@ Century bhai,
Your quote is valid for selecting the Current Boss OUT OF 'Ex- Bosses'. Even in your case/eg. you mentioned picking 1 out of the 4 former bosses. However, there's no such concept of
future_boss.. [future can be prefixed to canadian only
]
@ saira's situation is different.. she's getting a Letter from someone who was never her boss during her stint with the co. This person joined at her Manager level Only after she left the co.
I personally don't think it makes sense to get a letter from somebody whom you have never worked with (in the past or present) .. with any Employer.