As a further refinement, I just rewrote the function from scratch so that it's a little smaller and much faster:
void LCD4Bit::cursorTo(int line_num, int x){
commandWrite(CMD_HOME);
int row_address[] = {0x00,0x40,0x14,0x54}; //DDRAM addresses for lines 1 to 4
commandWrite(row_address[line_num-1] + x + 128); //128 to set high bit for command
}