There is a clause in the Net Profits Royalty information that defines Exploration Costs as those expenses incurred in the three year period prior to a Production Decision being reached. That would seem to limit eligible expenses prior to deciding to build to 3-years at least. Then they get into Capital Costs from Production Decision to the Completion Date.

There is probably a trigger somehow in the expenditure requirements part of a contract that would squeeze Teck into the 4-year time period.