Sharps, in his second spell with the club, spent the second half of last season on loan at Burton, former Bolton right-back Hunt was at Accrington, while Ainsworth enjoyed a spell with Aldershot.
Odejayi spent the entire campaign with the Millers but struggled for game time, making only 17 starts in the npower League Two promotion-winning campaign.
All four players are entering the final year of their contract and have been told they do not feature in boss Steve Evans' plans.
"At this stage of the summer, we have a job to get us prepared for life in League One," Evans told the club's official website.
"What comes with that are difficult decisions when viewing our squad in its entirety and the options within it.
"We must consider the targets we're aiming to recruit and when I looked at the big picture and after much consideration I have decided that the aforementioned players have to be made available for transfer with immediate effect."