8-9 days enough for 4 islands
Best season: weather is best from mid March to April.
Best way to see all islands is taking a cruise. You can see Kauaii, Hawaii (both sides of island) , Maui, Honolulu (only cover Peral Harbor and downtown) in 7 days and spend an extra couple of days in Honolulu since PCC usually takes a whole day. Then spend another day take blue line (I think) for an around the island tour.
Best way to see all islands is taking a cruise. You can see Kauaii, Hawaii (both sides of island) , Maui, Honolulu (only cover Peral Harbor and downtown) in 7 days and spend an extra couple of days in Honolulu since PCC usually takes a whole day. Then spend another day take blue line (I think) for an around the island tour.