However big photos 4Mb taken on a digital SLR camera can't be reduced without losing picture quality!!!!!!
I have some differbegging to do here, you CAN reduce the size of a 4mpx photograph without loss of quality by upping the resolution of your final resize. for instance on photoshop if you use image>image size, don't just reduce the size in cms or pixels, go down to the bottom and up the resolution. it's normally around 72dpi for a photograph to be used on the web. if your picture starts off at 60cms wide and you reduce it to 30cms for instance, up the resolution at the same time. you ndont necessarily need to double the resolution, i find anything between 96 and 144 dpi works well. try a few, check them you'll eventually find one that works for you which is a balance between size and resolution.
although bearing in mind when people look at photos on the web the quality is governed not by the resolution of the photograph but by the quality and resolution of the monitor used so anything above 96dpi you wont see the benefiot unless you're viewing it in print format.
I have a 10mpx camera. online there's little difference in the quality of lesser pixelation, but in print i see the difference so much.
hope this helps.
I can also give instructions for how to do this on GIMP and fotostation too.