Re: Oh, go on then
3-4mm - that's nothing.
My first robotic surgery project was for automated prostate surgery.
For that a resectoscope is passed up the urethra through the only natural keyhole. Not only does that contain the 'scope (which could be swapped for an ultrasound probe) but also the cutting equipment and irrigation channels. The whole assembly was housed in a stainless steel tube approximately 10mm in diameter.
And don't ask about the trans-rectal ultrasound probe (which goes in the other, larger, keyhole) used to guide a needle for prostate biopsies - that would put a lot of Ann Summers products to shame.