1) Is it permissible to recite a Non-wajib Salat in units of four rakat or is it that all Mustahab salat has to recited in two unit rakat.

2) If someone has made niyyat of 4 or 8 rikat Shukrana namaz then they should do it in units of 2 rakat?

1) Mustahab salaat is always recited in units of 2 rakats.

2) Yes