Get a cheap SSD for a boot drive. I think your only option given your constraints and your goal is use all three in a RAIDZ1. There's no real problem with using different kinds of drives in one pool. With this setup though you'll lose the use of 2TB of capacity on the 4TB drive. 2x2TB + 1x4TB won't work because you'll have no redundancy on the 4TB.
Edit: just realized you meant the 4tb could be mirrored to the 2+2 configured as a striped array. I think that could work, just have to see what truenas supports setting up. That would still meet your criteria of being able to lose one of them. Perhaps the ease of rebuilding a mirror compared to a RAIDZ1 would somewhat cancel out the doubled risk of the 2+2 failing. You'd only have 50% capacity, which in terms of number of usable tb the same as the raidz1 I suggested above.
I think personally I'd go RAIDZ1 as it seems more straight forward to set up but both of these options work.