As it is in many ux related questions it depends on your audience:
I consider the question style more suitable for users, that are not familiar with the form – or with web forms in general. You kind of take the user by the hand – which can be a good idea for that kind of audience, but might be a bad idea for 'power users'.
On the other hand I suppose the question style would rather soon get a user's nerves, once she has to fill out your form on a regular basis. For that kind of use case you should avoid questions and use 'statements' instead. 'Power users' will appreciate everything that makes their work easier – in this case short statements.
ps: does the form really have to spread over 4 pages? If if your audience consists of power users you should probably try to fit everything in one page…
As a rule, I try to keep labels as short as possible while remaining immediately clear.
Sometimes that's a single word or two. Sometimes it's a question. The idea is to help the user along by making it as straightforward for them as possible.
The best way to determine the clarity of your labelling is through observing users. Try getting two or three people off the street, give them a scenario and watch them fill out the form. Watch where they stumble and ask them to think out loud as they go about it. The approach is described in Steve Krug's Don't Make Me Think.
I would recommend "Budget" instead of "What is the budget?". If you ever decide to translate the form into multiple languages, it will avoid translation errors and probably be cheaper / less time consuming. Not to mention reducing the size of labels for languages like French.