Variables' variable names

Hello, all
Look at the following input scripts:

variable a index 2 4
variable b$a equal 3
next a
variable b$a equal 5

Up to here I have two variables include : b2=3 , b4=5
Please tell me how I should invoke b2 and b4 without using their direct name.
I mean when I use below codes:

variable c index 2 4
variable d$c equal ${b2}
next c
variable d$c equal ${b4}

It’s run correctly but when I try

variable c index 2 4
variable d$c equal b$c
next c
variable d$c equal b$c

d2 gets equal to b2 not the value of b2. Either d4 is.
Or when I try this one

variable c index 2 4
variable d$c equal ${b$c}
next c
variable d$c equal ${b$c}

it leads to an error.

Thank You.
Reza