If a 4 is rolled, you will get paid $50 (and you would get your $25 Buy bet back). So you throw the dealer $26, and they put $25 on the 4, and will put the BUY chip on top of your bet. When you get to the table, ask the dealer if the Buy 4/10 vig is 'up front' or 'on the win'. If you don't want to pay the vig, you can get your $25 buy bet back, and the total amount you paid was $1. But you now have to pay the $1 vig again, IF you want to have another Buy 4 bet of $25. If the vig is collected up front, that means you pay $1 when you make the bet (ie you pay $26 to make a $25 Buy 4 bet). If you want to make the bet again, a new vig will apply.

The Buy 4 bet is essentially a bet that ends when it is resolved. You only pay the vig on the Win, same as the 4/10.' 'Sweet murgatoids, I love the rules here!' 'Uh, excuse me sir, but you just paid me $1 too much.' 'No sir, that is the correct pay for a Buy 5.' 'Uh, I don't Buy the 5, that is a bad bet.' 'Not here sir. It is a little disorienting the first time you make a $25 Place 5 bet, and the dealer pays you $36, instead of $35. In this case, it does pay better to take the Buy bet, and in fact, the casinos all make it an automatic buy for you. In Tunica and Biloxi, MS they do allow you to Buy the 5 & 9, and only pay the vig on the win. So you will get paid better when it hits, but you will lose more when it loses, and the extra losses will overcome the wins, resulting in less money for you overall. The HE on the Buy 5,9 is 2% when paid on win only. The HE on the Buy 5,9 is 5% when paid up front.

However, that means every time you lose the bet, you are losing $21 instead of $20. So you are betting $21 to win $30 (plus it will cost you another $1 to put the bet up again).

On the 5 & 9 almost every casino takes that $1 up front. I'll let one of the math gurus put the math out here for you, but the part you're missing is when that $1 vig is collected.

Buying the 6 and 8 in craps