It's very tight. Would the 4 days include the day of arrival and departure? If so, then I think no chance of fitting it all in. If not, when would your arrival/departure be?

Eg you could do:

Monday AM arrive Hanoi
Monday PM City tour
Tuesday AM go to Ha Long bay - night on boat
Wednesday AM return to Hanoi
Wednesday PM fly to Saigon
Thursday City tour
Friday AM leave Saigon

Personally on such a short trip I'd skip Ha Long bay, unless you're really into scenery.

Look at the recent threads on visa on arrival and the providers you can use. It's expensive and a hassle via the London embassy. Don't hesitate to take GBP cash and change directly into VND. No need to mess around with USD unless you happen to have them anyway.