Making pool with mixed drives

Hello I have a 4tb and 2 2tb HDD’s I want to be able to combine the 2 2tb hdd to make a 4tb so I can have a 4tb pool. Is this possible?

Short answer no.

Longer answer, TrueNAS does not directly support this, and any attempt to do so can cause later support issues. (Specifically when replacing the 4TB disk, you will have 2 vDevs degraded, leading to potential to “forget” to replace both, only one.)

ZFS, and by extension TrueNAS, are not the most flexible NAS solution. TrueNAS was originally developed for the Enterprise Data Center market and as such, reliability and repairability were more important than supporting odd configurations that don’t generally come up in a Data Center configuration.

All that said, you can create a 3 disk RAID-Z1 pool which will give 4TB of space. (Less for overhead and 80% suggested max usage...)

Basically ZFS will take the 4TB drive and treat it as a 2TB drive. Giving you roughly:
2TB data + 2TB data + 2TB parity
Then, if / when money permits, you can replace the 2 x 2TB drives with 4TB, (or larger), and then you can grow your pool to 8TB.

Of course, 2TB and larger disks are not suggested for single parity configurations due to the remote chance of a read error during disk replacement. So backups are always suggested.