Your cost base on the 5000 shares that you are selling will be $2.00 (not $4.00) for an increase of $4.00 per share.

But your cost base for the remaining 10,000 is now $2.00 (not the $1.00 you originally pd).

So you pay more tax on the 5000 but less on the 10,000. The overall tax you would pay is the same with either calculation.

It's 'Pay me now or pay me later'