There are two things to consider.
Should seniority be the sole criteria?
Musharraf was junior to Ali Quli Khan, but so was Zia to 3 others. Yahya Khan got rapid promotions and was Brigadier at the tender age of 34, when most others are lucky to become Lt Col’s. Eisenhower was only battalion commander in 1939 when WW2 started but was appointed Supreme Commander of the Allied Expeditionary Force for North Africa in November 1942. If seniority was the only criterion, Napoleon would have never made a General at the age of 26.
If we are to avoid mediocre such as Musa Khan or the cowards like Niazi heading critical military posts; ‘Merit’ should always take precedence when it comes to promotion to the top posts in the Armed Services as well as in the Corporate sector
Theoretically all serving Generals from Artillery, Armour or Infantry; if promoted on merit; are capable of COAS job once they reach 3 star rank. Therefore it is the question if selecting best among the best. We wouldn’t know how well or bad the person will perform until he is promoted.
However, merit is a ‘Subjective’ criterion and if someone is promoted out of turn, it is not his fault but onus is on the one who promotes him. Hence ZA Bhutto who promoted Zia & Nawaz Sharif who promoted Musharraf took wrong decisions and suffered the consequences.
Should the tenure of the 4 Star General be extended to 4 years?
Understand that under the present system, a major General retires at 57 & Lt General at 58 or after 4 years in the rank whichever is earlier. For example if a major General is promoted to 3 star rank at the age of 56, he will retire after 2 years only, whereas if he were to become a Lt General at 52, he would retire at 56 unless he is promoted to 4 star rank.
I am personally against extending the tenure of serving generals, because it would deprive some other deserving General chance of becoming COAS. Additionally, each new Chief brings new ideas and extension can translate into more of the same as happened with Gen Kiyani which delayed Operation Zarbe Azb unnecessarily.