Tell us more about your marathon. Were you running 20 min 5ks at the time you did the 4hour marathon ?
Did your pace fall off ? Did you cramp ? Or was it an even ish pace.
A 20 min 5k should get you something like a 3hr 20 marathon - but only if you train correctly for it. If you arent getting in the long steady runs - then you can forget it.
Dont be tempted to run your long runs fast. All you achieve by doing that is tiring yourself out - remember - the long runs are done on tired legs and you need to be able to carry on the next weeks training too.