It is usually 4 weeks from the day of the neuter for the healing to be complete and the hormones to die down for good. Often, later in that month, there is a final surge of hormones before they stop flaring up. That won’t cause a pregnancy in your female if she is not spayed, but will cause potential aggression in his behavior. If your female is not spayed yet, keep her away from him for 4-6 weeks to be on the safe side, but I’m assuming she is already if she’s bonded to Ralph.
So, one month is the waiting time. But meanwhile, I’d suggest using surrogate toy bunnies, one for each of them, during that period. You can set each one in their habitats to get the smell of each on them. Then put Ralph and Rosie’s surrogate in with Rowan, and Rowan’s in with Ralph and Rosie. If any have an adverse reaction, they will take it out on the surrogate, and then get used to the others’ smell. I’d say leave surrogates in for a week, then start switching.