Hello Stephen and thank you for the enquiry. We have discussed the USB 3.0 cable length issue as there are numerous workshops using the scope in exactly the same fashion with a boom.
At present, 1.8 meter is the length available and you are right that 3.0 meter should suffice.
The 4425 scope is USB 2.0 and 1.1 compatible but a 4.5 meter USB cable would exceed the recommended standard cable length for USB 3.0 of 3 meters.
I have used the 4.5 meter Pico blue USB 2.0 cable with my 4425 scope and experienced no issue, but we should err on the side of caution here to protect the integrity of the signal when taking advantage of faster screen update speeds.
One option would be to include a powered USB hub to connect 2 x 1.8 meter Pico blue USB 3.0 leads together as this would most certainly allow the scope to reach the end of your boom, but would include an additional power supply for your hub.
I will continue to request a 3.0 meter option here at Pico and will feedback when I have some news.
I hope this helps, take care.....Steve