I have a template of four single chains, however in the target
(single-chain) structure the chain segments are in a different order
...
*Is it possible to somehow specify this correctly in the alignment
file?*
No - your two options are (as you correctly stated) to reorder the input
PDB or to use multiple templates (losing interchain restraints in the
latter case).
That said, it's a little unusual to build a single chain model using a
multi-chain template. What are you trying to do?