Bedford Blues have brought in flanker Jon Fisher, prop Nico Steenkamp, centre Henry Staff and wing Mark Kohler.
Former England Under-20 player Fisher, 23, arrives from London Irish where he made 19 appearances in four seasons.
South African Steenkamp, who served a three-month ban earlier this year for consuming a banned stimulant, switches from Rotherham.
While Staff, 20, comes in from Saracens and 21-year-old Bedfordshire University student Kohler leaves Luton.
"Henry has great power and strength and we are all looking forward to seeing him competing in the centres week-in week-out," said director of rugby Mike Rayer.
"Jon has arrived with a big reputation and a glowing reference from London Irish.
"Mark has shown what a great talent he is on the wing and has been on our radar for some time.
"He was on trial last year but suffered an unfortunate ankle injury which ruled him out for some time."
Former Peterborough forward Steenkamp sat out the end of the Championship season after testing positive for methylhexaneamine.
The Rugby Football Union accepted Steenkamp did not know the high energy drink he was taking contained the banned substance.
"Nico is still a young player who has a wealth of experience across several different tiers of rugby," Rayer added.
"He may have hit the headlines for the wrong reasons last season, but he took his punishment on the chin and is raring to get back into action in a Blues shirt and we are all excited in seeing what he can do to bolster the front row."