clkernel -3 recognizes the GPU RAM size and switches automatically the cards with 4GB to clkernel -1. Sou you will not have a problem by simply passing clkernel -3. If you like your 4GB card to use clkernel -2 you have to pass clkernel 333333332 if your 4GB card is with index 9, otherwise you have to put the '2' on a position equal to the 4GB card index. You can find the index of the GPU in PM's log when it initially lists the available GPUs.