Pompey boss started his preparations for the 2016-17 season with a clear-out of his current squad.
Keeper Brian Murphy was told he would not be offered a new deal and was free to look for a new club.
Recruited only last summer, Kal Naismith and Adam McGurk were told they had been place on the transfer list alongside the out-of-favour Matt Tubbs.
Meanwhile, youngsters Chad Field and Snorre Nielson were also told their Fratton Park careers were at an end.
The four first-team deaprtures will create space and funds for Cook to bolster his ranks as he plots a route out of League Two next term.
Here, we have a look at the brief Pompey careers of Murphy, Naismith, Barton and Tubbs.
After arriving from QPR in a free transfer last summer, the Irishman was quickly installed as Paul Cook’s No1.
However, he never truly impressed the Fratton faithful, with injury also hampering his progress.
He returned to the first-team after a three-month injury absence as a second-half substitute against Wimbledon last month.
But once again injury took its toll, with Murphy unable to play a part in the Blues’ play-off campaing with Plymouth.
Made 25 appearances for the club in the 2015-16 season.
Signed for an undisclosed fee on three-year deal after leaving Accrington last season.
Made Pompey debut in 2-0 Johnstone’s Paint Trophy defeat at Exeter and made a total of 22 appearances for the Blues.
Joined Hartlepool on an emergency loan in November, playing four games for the Pools without scoring.
Returned to Pompey in January and scored in 2-0 win at Stevenage - his first goal for the club.
Scored in Pompey’s last two league games of the season but was an unused substitute in the 1-0 play-off defeat at Plymouth.
His last Blues appearance came as a sub in the first-leg clash at Fratton Park.
Moved to Fratton Park on a free transfer after leaving Burton Albion, signing a two-year deal.
Scored six goals in 22 appearances under Paul Cook, including a 35-yarder in the 1-1 draw at Oxford in September and both goals in the FA Cup victory against Macclesfield.
Injuries hampered his Blues progress, though, and was unable to establish himself as a regular in the team.
Came on as a sub in both play-off games against Plymouth but made little impact.
The striker moved to Pompey in January 2015 under former boss Andy Awford, signing a two-and-a-half-year deal.
He made an immediate impact at Fratton Park, scoring nine goals in 23 outings as he helped save the Blues from relegation from the Football League.
He also ended the season as League Two’s golden boot winner.
The diminutive front man netted five times under new boss Paul Cook by mid-October but soon fell out of favour and was loaned out to Eastleigh in February.
Tubbs notched only two goals in 16 appearances for the Spitfites in the National Premier.
And despite returning to Pompey before their play-off double-header with Plymouth, he was not included in any of the match-day squads.
Tubbs last outing for the Blues was the 2-1 defeat at Notts County on October 31.