Checking the English version of the regulations (as published by Roy and dated 2005) a producer intending to make an LBV has to submit his initial samples in the fourth year following harvest, the earliest date being March 1st. He then has to give the IVDP 15 days notice of intention to bottle, presumably to allow time for the samples to be tested.I believe that "4 to 6 years" is our shorthand as Port fanatics, and not the actual regulation.
This suggests that the bottling run could be made mid March if the producer wanted to be quick off the mark, but it seems that none of them do. The last possible bottling date is Dec 31st of the sixth year following vintage.