Depends....
..... on the wind range that you're looking at. A blade #3 up to 35kts and #4 to about 45 or 50. After that I'd start thinking about bare poles. Just out of curiosity, where are you planning to sail to? We use our #3 several times a year, but have never had a need for a #4. If you're a cruiser, then I would consider changing sails early. A headsail change with a racing crew is a routine thing, but changing headsails in 25+ knots can become a handful for anyone.