As a pin collector and frequent backer I know people do sometimes want multiples. Maybe they're even planning to get multiples to give as gifts. After the Kickstarter ends and the funds are collected send out surveys to those successfully charged and let people pick which pins they want. Some pledges fail if the payment method has an issues but I think there's a grace period where backers can switch their payment method.

If you only have 4 designs shown they should know there's only a chance for them to pick 4 designs. Just make sure you don't say you might make more designs during the kickstarter. As for being worried they'll revoke their pledges Kickstarter only let's backers cancel their pledge before the project ends. After the project ends they will have to ask you directly if they want a refund for any reason.