At 3.8 your aneurysm is very small, far from any concern in my mind. The main issue with any exercise is if it raises your BP, we know from many experts that holding your breath while exerting heavily raises your BP , in your case you are talking about breathing rhythm in an aerobic exercise, my reasoning tells me it should not rise significantly, if at all, but someone that has done studies with aerobic exercises would know the answer. I can tell you that after swimming or biking regularly my BP actually lowers (not immediately but when I take it in the evenings), but of course I don't know what happens during.

My aneurysm is already repaired (5.2 cm repaired in 2020, at 54 years old) , but I do get checked every year as I have a bicuspid aortic valve and they deteriorate with age. I exercise regularly and that includes high intensity 1 hr spinning classes, I get my HR to 160. During my check ups my cardiologist has told me whatever I'm doing is slowing down the valve deterioration, and he has done studies on exercise and aneurysms (the cardiologist in the video I have posted). He also told me though that although already repaired, controlling BP is important as in some cases dilations may occur at the point where the graft and the aorta were joined, or in my case anywhere else as I seem to have a genetic propensity.

On the other hand, being so active and maintaining your body in great shape will pay dividends if you ever need to have your aneurysm repaired, but at the size it is right now, that may never happen. I am sure your Drs are monitoring the growth rate, and that in my mind would be a point of reference.

Sorry I couldn't give you a straight answer, if I find more info I will post it