You need to create postage discounts and apply them to the listings in question, but you options are limited and it's not quite possible to do 1 or 2 for $1.20, then 3 or 4 for $2.40, as your options are limited to either adding a pre-set amount for each additional purchase, discounting by a pre-set amount for each additional purchase, utilising a postage cap (i.e. maximum amount payable), or free postage if a certain dollar value is reached or a certain quantity is purchased.

In this case, you may want to split the difference and say add 40c for each additional purchase - if someone buys two, they'll be charges $1.60, if someone bys 3 it will be $2.00, and 4 @ $2.40 - so basically you'd "win some, lose some" but hopefully even out. The only other option is to invoice manually post-sale.

You can also use two different rules to do something like 25% off the postage total for each additional item up to a maximum of X amount.

Be sure that, if you create postage rules, you don't apply them to other listings wherre it wouldn't be appropriate, as sometimes the box will be auto-checked.