We have a 98 GS4. Mileage is at 38k. I know what the manual says..but is there anything that absolutely has to be done at the 40k and 45k mileage? Thanks!

If you have done your 30,000-mile service then not much for 40K service. Replace engine oil, filter, rotate tires and the rest just inspect various components such as air filter, brakes, belts, hoses, fluid levels...