A FOUR-BED convalescent home will be set up soon for patients who have had bone marrow transplants at Queen Mary Hospital. Three flats have been earmarked at Wah Kwai Estate near the hospital, where doctors of the Bone Marrow Transplant Unit can watch patients while giving them some semblance of home life. The cost of establishing the homes, estimated at about $1.5 million, will come from this year's Operation Santa Claus, conducted by RTHK Radio 3 and the South China Morning Post. Patients who have undergone bone marrow transplants need a controlled, sterile environment because of the risk of infection. This can involve stays in hospital of up to 50 days - precious time which could be given to others in need of the vital transplant. ''It's a no-win situation,'' said Dr Raymond Liang, secretary of the Hong Kong Marrow Match Foundation. The Queen Mary Hospital Bone Marrow Transplant Unit performs 90 per cent of all transplants in Hong Kong and can manage about 120 cases a year. Patients stay for between four to eight weeks after the transplant and need to return every one to three days for up to three months after they are discharged. This causes problems for more than half of the patients who live in Kowloon and the New Territories because, in addition to risk of infections, they suffer severe motion sickness because of medication. Being allowed to return to a home-like atmosphere as early as possible would improve the efficiency of the service and directly benefit an average of 15 to 20 patients waiting for transplants. Information from nurses and patients show that an average of four to six patients in 30 follow-up cases require this sort of temporary accommodation. The plan calls for the setting up of ''small group homes'', consisting of a cluster of self-contained units of about 200 square feet to be rented from the Housing Department. A part-time manager and part-time domestic helpers will be employed to run the home and relatives will be encouraged to stay with patients to help them set up routines. The home will be managed by the Queen Mary Hospital Patients Resource Centre with input from the Hong Kong Marrow Match Foundation and the Hong Kong Bone Marrow Transplant Patients Association. The $1.5 million needed could fund three units for three years.