Chris Woakes claimed five wickets while batsmen Root and Morgan hit half-centuries to help the hosts defeat Pakistan by 54 runs.
Jason Roy and Ben Stokes star as England complete a series win over Pakistan with a tense victory in fourth ODI on Friday.
Jonny Bairstow Century was the centrepiece of England six-wicket win over Pakistan in the third one-day international at Bristol on Tuesday.
England beat Pakistan by 12 runs in Southampton, with Jos Buttler scoring the second-fastest century by an England batsman in ODIs.
Jofra Archer makes impact as England and Pakistan get off to damp start.
Chasing 174 to win, Morgan fifty helped the home side get home in the final over.
Josh Little rattles England before fellow debutant Ben Foakes comes to the rescue.
Ireland bring experienced wicketkeeper and batsman Gary Wilson back into their ODI squad.
Ben Foakes joined England squad for the one-off ODI and one T20I against Ireland against Pakistan.
England do not pick Jofra Archer in their preliminary 15-man World Cup squad but name him in squads for one-day games against Pakistan and Ireland.