Welcome to the forums!
Not much. TrueNAS and ZFS does not have many options for mixed disk sizes. This is basically your main / only choice;
240GB - boot disk
2 x 2TB in a Mirror pool
4TB in separate pool, as unprotected storage, (like a backup of your Mirrored pool or media backed up elsewhere)
A few people come to TrueNAS, (formerly FreeNAS, but TrueNAS Core & SCALE are free to use), with just an idea for a NAS. TrueNAS and underlying ZFS do not support well, thrown together hardware. ZFS is designed for high reliability, but just using left over hardware may reduce reliability noticeably. Some consumer disk drives are actually in-compatible with ZFS because of various reasons, (TLER, SMR, spin down, etc...).
Many of us in the forums want reliability, recover-ability and data verification that ZFS gives us. So we tend to design our hardware to accomplish those goals, make our hardware choices conservative. And having to select hardware specifically for the task of a ZFS based NAS.