So get 4 on making Concierge, Concierge lasts at least 12 months depending on your expiry date, so you are Concierge until the expiry date and 12 months thereafter.

Come the next year if you renew at Concierge you get the 4 upgrade once you cross 1051 again, so at that point you can in theory have 8 vouchers active for a period

You only get 4 vouchers for each time you earn 1051 from 0

So example, expiry date June 2023 shows on account
I earn 1051 by March 2023
I have Concierge status until June 2024 and 4 vouchers valid 12 months
June 2023 your tier credits reset to zero
By Feb 2024 you earn 1051 again
I have Concierge status until June 2025 and 4 new vouchers valid 12 months