It depends on how strong your vision is of the character you want, and how comfortable you are moving the dials around.  I had worked up four major characters of my own that were original face morphs with V3; when I changed my efforts over to V4, it took me maybe a week to ten days for each of them to convert to something pretty close, although a couple of the characters evolved into something rather different because my own vision of those characters changed, not so much because I couldn't reproduce them exactly.  All of my characters also became a bit more evolved and stylized to my own internal vision.

If you have a good reason to convert to V4 (I did, superior bending, plus I really liked the materials settings for the high rez material set) then I would suggest you just go ahead and buy the figure + morphs, and be prepared to take advantage of the 100% money-back guarantee if you are not satisfied.

Good advantages that V4 has over V3:
Morphforms really are a nice workflow improvement if you don't use canned poses and do all your poses yourself (and I do).
Better bending at the shoulders, back, hips and buttocks, although there are some new bending problems that were not serious issues with V3
Closer to a natural starting point, although V4 is still quite a bit taller than average
Lower poly count by about 10k

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