Hi Emaldon - I think you can do it for up to 12 weeks without a "rest" so you should be fine to just do it for 4 weeks. I did it for 6 weeks prior to my holiday and lost 20 pounds! First day back today and straight back on my diet again as I have a few more pounds to go...
Good luck!
You can do it for 12 weeks, then a week on SS+, then go back to it if you haven't achieved your target weight. (That's what I'll be doing I think - I doubt whether I'll get to my target weight in 12 weeks, but you never know! )
VLCDs are meant to be a way safe way for people with a large amount of weight to lose it quickly. It is not a good idea to use it as a kick-start or quick fix, mainly because if you do it for two weeks then return to eating 'normally' you may find you have a large water gain which negates anything lost in those 2 weeks.