Short answer no.
Next answer, really no.
You should read up on ZFS terminology. You seem to be using "pool" when you mean vDev.
Also, you can have a ZFS pool with 2 vDevs, 1 of which consisting of 4 x 4TB in a RAID-Z1, and the other consisting of 4 x 2TB in a RAID-Z1. The space from both vDevs will be added together into a single pool. However, it's not a good idea to use RAID-Z1 with disks larger that 1TB.
Long answer, it depends on exactly what you mean.
If combining 2 x 2TB disks in to a 4TB psuedo disk, and using it as part of a ZFS vDev made up of 4TB disks & 4TB psuedo disks, then yes, it's possible.
Is it recommended? NO.
Is that going to be reliable? NO.
Can you do it from the GUI? NO.
Basically if this was your question, and you have to ask the question, you don't have the skills to actually do it. (And support it after...) Sorry. This comes up every now and then.