It's a fact that when an amplifier is bridged with a single driver on it, each channel "sees" half of the driver's resistance. With an 8 ohm driver, each channel will see 4ohms. Now factor in an additional 4 ohm load per channel, without any passive crossover between the sub and the components, and the final load will be 2ohms per channel. The only way to keep the amplifier at 4ohms per channel is to make certain that the sub and the components do not overlap for any part of the frequency response and that will take careful designing of a passive filter. I didn't see any passive filter components in the drawing so I'm forced to assume there aren't any, in which case, the final load is 2 ohms per channel.