Open deck means the cylinders have one big coolant passage all the way around the deck and it is not supported. 4g63 is a closed deck, with just the coolant passages open. A semi-closed deck will have supports going from the cylinder to the block, but still fairly open. It should be fine for high HP though from that aspect. That is why the Honda guys put a block guard in, like so:
That turns an open deck into a semi closed deck, the 4B11T will come with something like that installed from the factory.
Threaded Mode