The 4x4 on mine is manual so there was nothing but the cig lighter and heated seat buttons below the factory HU. I moved the heated seat buttons to the floor console where I have 3 rectangular pop-outs (one now). You may be able to replace the 4x4 button with another nissan button that could fit in one of the rectangular pop-outs. <<Just took another look at your pics and it looks like more than a simple button - so that will not work.
Another thing to consider is that the space above the vent is perfect for a GPS and Wi-Fi-Antenna. There is plenty of room - I just used heavy duty velcro to mount them - I get good signal on the the wi-fi, have not really tested the GPS yet except with hyper term and it appears to have a signal.
One word of warning if you choose to drop the Bose HU. I replaced mine with a Clarion and am having a difficult time getting good sound which is as loud as before. I am using a poor quality (IMO) Line Output Converter which I am looking to replace with a LOC from davidnavone.com - the problem is that the Bose speakers and amps are not made to play nice with aftermarket head units. I am somewhat commited to it now tho

If you choose to keep the Bose HU, you may want to search for a AUX input adapter - I think
www.logjamelectronics.com told me there was one in the works recently.
The cable to the AC control is short - too short to relocate without splicing in a new section. Here are some pics of the duct that I made from a vaccum attachment, for cooling the PC. The last one shows the AC control hanging down - you can see the cable is not that long: