Unfortunately not, the 4X4 system on an RX300 is permenantly engaged. Yes, there would be huge fuel savings if that were possuble! :-( The alternative is to follow me into an RX450h, but then you'd have to weight the purchase costs against the fuel savings. In UK mpg terms my Hybrid is regularly giving 32-34 mpg, consistant 28 around the city & have seen as high as 37mpg on a few long runs. That said if you use all the performance all the time it is easy to get down to 24-25! ;-)
Cheers, to be honest the mileage she does (100-150 miles pw) doesn't warrant the outlay on a 400h. It's only used around town so averaging about 18mpg, she doesn't want to give up the comforts on the rx so think the best option re running costs is to look at lpg.