LAST year's ideal home is already outdated, as visitors to this year's Electrical Home Appliances Expo and Ideal Home Expo will see. The fourth fair, which opened today and will run until Monday, has attracted 175 exhibitors who will take 485 booths. The products on display are proof of how quickly designs change. Since last year's event, hundreds of new home products and electrical appliances have carved their niche in the market; many of them with a coveted patent. This growth is reflected in the rise in the number of exhibitors and booths. With about 35 more exhibitors setting up an additional 15 booths, visitors can expect to be kept up-to-date with trends at home and overseas. And nothing is too small to come under the scrutiny of designers competing to become a ''household name''. One of the smaller stars of this year's event will be the semi-conductor toothbrush from Health Co in Kowloon. One of the bigger stars will be the moveable house. And, in between these extremes, are literally thousands of time-saving, labour-saving devices and those which simply add to the comforts of home. Broadly speaking, the exhibits will cover household appliances, furniture, lighting, fixtures, home furnishings, bathroom accessories, fabrics and upholstery, decorative items and kitchenware. These will be displayed over levels five and seven of the Hongkong Convention and Exhibition Centre and cover 13,500 square metres. New products always create keen interest. This year, stands will exhibit a broad range of state-of-the-art design and technology, including water purifiers, ''nodor'' environmental protection products, polycarbonate aluminium shutters, double-deck range hood and a LCD viewcam. The home appliances expo has been organised by the Hongkong Trade Development Council (TDC) working with the Hongkong and Kowloon Electrical Appliances Merchants' Association (HKKEAMA). The home expo has been put together by the TDC and is sponsored by the Hongkong Furniture Dealers and Decorators' General Association. The event was officially opened by the chairman of the Hongkong Tourist Association, Mr Martin Barrow, and the president of the HKKEAMA, Mr Leo Lee. In his opening address, Mr Barrow said that while the two fairs were different, they were complementary in the household supplies industry. ''Ideal Home Expo '93 provides ideas and supplies for furnishing and decorating the perfect home, while the Electrical Home Appliances Expo '93 provides the opportunity to equip the home with the latest in electrical conveniences,'' he said. Mr Barrow congratulated the TDC for putting together the two events. ''I would also like to thank the HKKEAMA for its invaluable contribution as co-organisers of the Electrical Home Appliances Expo,'' he said. ''Similarly, I thank the Hongkong Furniture Dealers and Decorators' Association for sponsoring the Ideal Home Expo.'' Mr Barrow said the support for the shows and their success was a reflection on the increasing prosperity of Hongkong's population. ''Although they are both relatively young events - the Electrical Home Appliances Expo has been running since 1990 and the Ideal Home Expo since 1991 - the two fairs are already gaining popularity and I am delighted to see that, this year, there are moreparticipants than previously,'' he said. ''Last year's fairs yielded business amounting to some $30.4 million and, with the increased number of exhibitors this year, the fair should be even more successful. ''There will certainly be a great deal to attract members of the public to the shows but, with thoughts of home, let us not forget our visitors. ''While they are still relatively few in number this year, we hope to attract greater interest for next year's and future fairs.'' Earlybirds to the four-day expos will receive mini-torches from Hongkong Telecom and Brandt. Doors open at 11 am today and 10.30 am over the next three days. The exhibition will close at 8 o'clock each night except Monday when it will end at 6 pm. Admission is $15.