Preeties - I flew with DS longhual (12 hours) by myself when he was 4 months old. Did it again when he was 7 months old and doing it again in 4 weeks when he is 18 months old. I can hand on heart tell you that 4 months was the easiest! If you are BF-ing, just feed on takeoff and landing to reduce the ear pressure. I was bottle feeding and tried to time it that way but never quite worked and actually, DS was one of those babies that didn't seem bothered by it.
On check-in, I would ask the check-in person to make sure you have a seat with the bassinet, it should be fine though if you already let them know you are travelling with a 4-month-old when booking. They don't provide infant seats as such as your baby won't get a seat, only you will and your baby will get the bassinet (which is like a carrycot and the baby will lie flat in it).
I have no idea why you get the window and middle seats. You are right in that it is a right pain for the other passengers. I was very lucky on the first flight. The seat in the middle was empty and the lady sitting by the aisle had just been to visit her daughter with a similar aged baby as DS so she turned out very helpful.
What I found a real pain was that if there are any turbulence, you are expected to take your baby in and out of the bassinet even if he/she is sleeping so be prepared for that! It depends on how strict the steward/ess is as well. On one flight, I was made to take DS out of the bassinet every single freaking time the seatbelt sign came on. On the return flight, the steward/ess pretty much left me to do whatever.
Do you have a sling? When we travelled with DS at 7 months old, we had an Ergo and we found that very useful as he would never sit still on our lap (you had to attach another lap belt to your own to go round your DS) but in a sling, he slept quite comfortably and the steward/ess left us alone as I guess he couldn't be strapped in any tighter than that!
How sensitive is your DS to light and sound? My DS was quite sensitive to light and sound and that was a slight problem in that the plane lights were never completely dimmed and people used the loo and slammed the overhead storage a lot! Take something breathable with you (like a muslin cloth or cellular blanket) that you can cover the bassinet with to help your DS sleep.
Last but not least, tell yourself it's only 11 hours and start counting down the moment you get on! You will survive just fine. Enjoy!