I assume you're referring to https://wizardofodds.com/games/blackjack-plus/
. In which case the advice is not to play it!
However there are a few Blackjack games that have a bonus for getting a 5-card hand, some of them stipulate it has to be a 21 and others just five non-bust cards (as here).
In most Blackjack games the number of cards in your hand don't matter so, ignoring card composition effects, your strategy is only based on your total and the dealer's up-card. So normally it would not matter whether you had A9 or A234 as usually you would stand on the soft 20. However in this game you are a guaranteed winner by hitting A234.
This affects your play with four card hands where usually you would stand. If you hit your chances of winning are solely dependent on whether you bust or not by drawing a 5th card rather than what the dealer gets. With lower totals hitting is better than standing.
This then affects 3-card hands as they might turn into hitting 4-card hands, which is why you always hit 3-card A25, 12s and most 13s. Interestingly it even means hitting AA against high-cards - as a sequence of small cards give you a good chance of a 5-card hand.