i can see why you want to use f/4.0 to achieve a dreamy feel, to "soften" the image. but i hope this is what you did because you wished to achieve the dreamy feel , rather than just letting the camera decide and correct its error later with processing. there is a subtle difference; in the former you know what you are doing, that displays a sort of familiarity with technicalities of photography, in the latter, you are using software to correct mistakes.
anyways - a dreamy feel *could* work here, but not with this scene, really. it simply does not add up.
perhaps you could see the links posted in this thread here if you are familiar with the concepts of exposure, etc.. if not, brush up on these, then visit the links eventually. i'm sure you can draw some inspiration.
thread link