When you've created your currencies-picker snippet, you need to make the following additions:
Define an additional CSS class:
Add your 3rd and 4th currencies:
Anything that's not on the far left or on the far right must have a class set to middle. Instead of the three-letter currency codes, you can use images or the currency symbols as content of the span elements. Only the data-currency attribute values matter here. The content of the span elements can be anything.