Parkinson, installed as Peter Jackson's replacement just after the season started last August, has so far had to work with the players he inherited, but is keen to mould his own squad in a bid to turn the club's fortunes around.
Defenders Marcel Seip, Rob Kozluk and Guy Branston, plus midfielder Lee Bullock are among those who will move on in the summer and Parkinson said: "It was a tough call.
"We felt it was time for Lee to move on. Lee's a good pro though and did well for us at times during the season. We wish him all the best and thank him for all that he's done for the club.
"Marcel's situation is slightly different because he's based in Plymouth and 'wanted to try and get something down that way.
"Kozzy's a good lad and made a contribution for us, but I feel I need to try and bring in a couple of younger full-backs. For the moment I've got to look elsewhere and so has he and I think Guy understands I've got to try and build my own squad up and put my own stamp on things."