Andrew and O'Connor move up from the Hughes Insurance Ulster Rugby Academy to development contracts, while McCall and Nelson extend their development contracts by an extra year.
All four players will then move to full-time professional contracts in 2016/17.
21 year-old, John Andrew, played in four of Ireland's U20 RBS 6 Nations matches in 2013. He made his first senior appearance for Ulster against Exeter in a pre-season game last August and has made four appearances for the province so far.
Ex Ireland U20s prop, Kyle McCall, joined the Academy in 2010. The 23 year-old made his debut for Ulster, against Leinster, in 2013 as a replacement in the win at the RDS. He has played three times for the senior side so far.
22 year-old, Peter Nelson, represented Ireland at U19 level and made his debut for Ulster on Boxing Day 2011, just a few months after he had completed his A Levels. He can play at fullback or on the wing and has 21 caps for Ulster.
Second-row, Alan O'Connor, has played 13 times for Ulster and made his debut for the province in a home win over Leinster in December 2012. The 22 year-old was awarded the Ulster Rugby Supporters Club, Jack Kyle Bursary, for the 2014/15 season.
Ulster's Team Manager, Bryn Cunningham said:
"All four players have shown significant improvements over the past year that should result in very promising futures in the professional game.
"Of the young hookers behind Best and Herring, Andrew, in his brief cameos with the senior team and performances for the Ravens, has shown he has all the credentials to be a top class hooker.
"The next two years should undoubtedly see Kyle McCall break through and challenge for a senior starting position. His work ethic, rugby intelligence and professionalism make him a star of the future for Ulster Rugby.
"Peter Nelson has developed his power and strength while becoming a more confident player at the highest level. With his attacking flair, pace and undoubted talent, he too has a bright future ahead of him with Ulster Rugby.
"Alan, due to injuries in the senior ranks in the early part of the season, was thrust into the first team where he excelled beyond expectation. A hard worker, he is sure to be an important part of the squad moving forward."
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