Peter,
There are some very good suggestions here, however it appears from your post that you already have the model built and flown. If you constructed the Amraam 4 without dual deployment, that means you epoxied the coupler into the upper bodytube. This will make it more difficult to incorporate, but not impossible. Please note that with the following suggestion, you only need to modify the coupler bulkhead plate...no additional tubes need added and the original coupler can remain in place.
I would suggest cutting a 2.1" hole in the bulkhead at the end of the coupler, which is glued to the upper body tube. You can use another CR to overlay it and draw the hole. This will convert the bulkhead into a centering ring with a 2.1" center hole. Next, you can glue an internal centering ring at the top of the coupler...a 3.9" to 2.1" coupler CR could be used. Then I would glue a piece of 2.1" body tube to "link" the two CR's. If you wish, you can glue a Piston strap to the side of the 2.1" tube and have it protrude out the top of the rocket so you can connect a piston.
All that remains is to build a Alt bay. A 2.1" coupler can be used with a 2.1" bulkhead plate at the upper end and a 3.9" bulkhead plate at the lower end. You can attach a eyebolt to the bottom 3.9" plate(to replace the original you cut out) along with a cheap BP canister made from PVC or copper tubing. A board can be built to slide inside the 2" Alt bay and two all thread rods running the length of the bay along the sides can bolt the end caps together.A PVC or Copper ejection canister can be bolted to the top 2" plate and with the unit assembled, it inserts into the 3.9" coupler and bolts in place.
I would use a couple 8-32 or 1/4-20 T-nuts inside the lower lip of the original bulkhead, which is now a coupler. This can be used to bolt the alt bay in place. I have attached a couple pics of similar bays I have built below.
Good luck and keep us up to date,
Carl
Building an altimeter bay-
https://www.rocketryforum.com/ctulanko/album/index.php?dir=tres/&startpic=40
Building a 1/2" copper tubing Ejection System-
https://www.rocketryforum.com/ctulanko/album/index.php?dir=tres/&startpic=50
View of what your bay could look like-
https://www.rocketryforum.com/ctulanko/album/index.php?display=tres/Tres 248.jpg