4名以上の利用時に割引って英語でなんて言うの？

On a discount voucher it is good if you can use as few words as possible to get the entire message across clearly. In this case that is possible as there is very little information necessary to include here. You could say, "Valid just once," or "Single Use Only" to indicate that the voucher is not for multiple visits and multiple discounts. It is quite common also to have an expiry date on any voucher: 'This offer expires on 31st January.'

You can use the following statement:- A one-time 20% discount is available to groups of four or more. This statement means that the discount is available one time only, the amount of the discount is 20% and it is available to groups of people with either four or more than four.

Most stores do offer specials during certain events such as the festive season. If yours is one of those stores, I assume you could be offering a special. So, you may write on your coupon: Groups of four or more pay 20% less. or Groups of four or more get a 20% discount.

"Groups of four or more will receive a 20% discount" This explains that groups that have more than 4 people will get a discount. "Valid for one use only" This explains that the coupon can only be used once. "20% Discount for a group of four or more." This is another way of explaining that groups with 4 or more people will get a discount.

You can write it either of these ways and it will be understood. Group indicates a number of people who are doing something together. Four or more shows that the minimum number of people cannot be less than four, but there is no limit to how big the group can be. Discount is a certain amount that you do not have to pay, in this case 20% of the total cost. You can also say 20% off which is also understood to be the amount that will be taken off of the final bill. Valid only once and one-time use only both state that this coupon can only be used one time.

The first part of the sentence, you are specifying that this is only for 'groups of four or more' (for four people, or for groups that have more than four people). The word 'recieve' is a formal way to say 'are given'. And '20% discount' is having 20% off of their bill. 'Valid only once' is the most common way in England to advertise that you can only use this coupon one time.

When writing on a coupon it is always good to use a few words to make the message clear and understandable groups of 4 or more means 4 or more people to say you can only use it once you could say 'valid only once' or 'single use only'

If you want to write on a coupon that groups of four or more receive a 20% discount, and that the coupon is only usable once, you can say: "20% discount for groups of four or more - valid one time only."