Finding the resistors value in a transistor circuit that uses a constant current source
If you assume that Vbe = 0.7v (not too far wrong with no other information), then you have R4 correctly calculated at 1.75k.

R5 is a different story. That's not a current mirror circuit. R5 can be anything from very small to quite large. If 'act as a current mirror' means we chose the Q1/R5 current be 400uA as well, then making the same 0.7 assumption leads to an R5 of (8-1.4)/400u = 16.5k.

If Q1 were replaced by a 1.75k resistor, that would be a current mirror, and changes in the current through R5 would be faithfully reflected to the output I3. As the voltage across Q1 varies little with changes in current, R5 could go up or down by a factor of 10 with very little change in the output current.