THERE are just four more days in 1993 - four days in which to make a real contribution to this year's Operation Santa Claus. The total may have tipped $1.8 million, mostly on the smaller donations of Hong Kong's generous community, but the RTHK-South China Morning Post campaign needs $5 million. Companies are back at work today. We need their cheques, TODAY! The youngsters who desperately need bone marrow donors, transplants and surgical convalescent facilities are depending on the generosity of Hong Kong businessmen at the end this incredibly prosperous 1993. Apart from a hefty cheque, made out to Operation Santa Claus, there may also be someone who wishes to give just a little bit extra to this worthy project. Every Wednesday you can register as a bone marrow donor by undergoing a simple blood test. All you have to do is go along to the Red Cross Blood Collection Centre at Shop 330, Omni Hong Kong Hotel Shopping Centre, Tsim Sha Tsui, between noon and 7 pm. Or, by donating $500 or more by credit card or cheque to the appeal, your name will automatically go into the draw for a trip for two to San Francisco on Singapore Airlines Raffles Class.