Again, you need to look at the bottle and see if you have 100mg/ml or 200mg/ml. It should be printed right on the label. Usually it is 200mg/ml but not always.
If it is 200mg/ml then 2ml = 400mg as you stated.
Perhaps your doctor is trying to do what is known as "front loading" that steroid users often do with some types of steroids to work around long half lives. But T cyp half life is somewhere between 5 to 8 days which isn't all that long. With a front-loading strategy, you would get big dosages initially and then fall back to the actual TRT maintenance level of somewhere around 150mg/week. Also, I almost forgot, one moderate risk to a dose as high as 400mg is a spike in Estradiol (E2). You don't want this. And if you confirm your dose was 400mg, you should mention this E2 potential spike issue to your doctor.
When I got my first shot (200mg) I felt the blastoff about 4 to 6 hours later. My T started that day though at 310 before the shot. If yours is really low I suppose it might take a bit longer before you feel it. Although, on the flip side, I would think if it is really low, a big 400mg shot should be felt pretty quickly.
And of course you must remember everybody's chemistry is different and some guys feel much more from the boost than others. If you have been terribly low for a long time, it might take a while for other systems in your body to come back to full life in order for you to realize the benefit.