They don't sound like any bare standard LEDs that I have ever come across (normal Vf starts about 1.9v and doesn't go much higher unless they are white ones). From the voltage range given, it sounds like they have a built-in current limiter effective for just that voltage range, and so will need some extra regulation, use diagram 3 in the link. If you have a 6 volt battery, use that to measure the current on one, you then have the numbers to put into the calculator.
Remember LEDs are polarity sensitive, connect them the wrong way round, they protest by dying instantly. If that information is all that the seller is offering, it is only half the story, the important stuff having been missed. If they each pull about, say, 25mA, 40 will pull about 1A, so about a 3 hour shine with them all lit at the same time.