I wouldn't add altums for a couple reasons, main ones being:
1. They're aggressive fish - despite being angels - in a tank that's only 4 feet long you'd face a few problems after an alpha angel established itself.
2. They get a lot bigger then regular angelfish. I wouldn't keep them in anything less than 3 feet tall, 6 feet long and 2 feet wide.
3. Altums are wild caught fish. Wild caught fish in general harbor parasites and possibly gill flukes that domestic strains have not been acclimated to in nature. But, altums are so delicate that it's a very high risk to try and treat them for parasites, and doing so has a good potential of killing them.
Not sure how many gallons that tank is, but if you'd let me know I could tell you how many discus. If it's a 55 then 6 tops, if it's a 75 then you could get away with 8 or 9.