# Julia 0.6, 28 24 bytes

~A=sum(rotr90.([A],0:3))

~A=sum(rotr90.([A],0:3)) #
~                        # redefine unary operator ~
A                       # function argument
[A]       # put input matrix A into a list with one element
0:3   # integer range from 0 to 3
rotr90.(   ,   )  # apply function rotr90 elementwise, expand singleton dimensions
rotr90.([A],0:3)  # yields list of rotated matrices:
# [rotr90(A,0), rotr90(A,1), rotr90(A,2), rotr90(A,3)]
sum(                )  # sum


# Julia 0.6, 28 24 bytes

~A=sum(rotr90.([A],0:3))


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~A=sum(rotr90.([A],0:3)) #
~                        # redefine unary operator ~
A                       # function argument
[A]       # put input matrix A into a list with one element
0:3   # integer range from 0 to 3
rotr90.(   ,   )  # apply function rotr90 elementwise, expand singleton dimensions
rotr90.([A],0:3)  # yields list of rotated matrices:
# [rotr90(A,0), rotr90(A,1), rotr90(A,2), rotr90(A,3)]
sum(                )  # sum


# Julia 0.6, 28 24 bytes

~A=sum(rotr90.([A],0:3))


Try it online!

~A=sum(rotr90.([A],0:3)) #
~                        # redefine unary operator ~
A                       # function argument
[A]       # put input matrix A into a list with one element
0:3   # integer range from 0 to 3
rotr90.(   ,   )  # apply function rotr90 elementwise, expand singleton dimensions
rotr90.([A],0:3)  # yields list of rotated matrices:
# [rotr90(A,0), rotr90(A,1), rotr90(A,2), rotr90(A,3)]
sum(                )  # sum

2 use element-wise function application to save 4bytes

# Julia 0.6, 2828 24 bytes

~A=sum(rotr90.(A[A],x)for x=00:3))


Try it online!

Redefines the unary operator ~.
An array comprehension builds a list of 4 rotated matrices, which is then summed.Try it online!

~A=sum(rotr90.([A],0:3)) #
~                        # redefine unary operator ~
A                       # function argument
[A]       # put input matrix A into a list with one element
0:3   # integer range from 0 to 3
rotr90.(   ,   )  # apply function rotr90 elementwise, expand singleton dimensions
rotr90.([A],0:3)  # yields list of rotated matrices:
# [rotr90(A,0), rotr90(A,1), rotr90(A,2), rotr90(A,3)]
sum(                )  # sum


# Julia 0.6, 28 bytes

~A=sum(rotr90(A,x)for x=0:3)


Try it online!

Redefines the unary operator ~.
An array comprehension builds a list of 4 rotated matrices, which is then summed.

# Julia 0.6, 28 24 bytes

~A=sum(rotr90.([A],0:3))


Try it online!

~A=sum(rotr90.([A],0:3)) #
~                        # redefine unary operator ~
A                       # function argument
[A]       # put input matrix A into a list with one element
0:3   # integer range from 0 to 3
rotr90.(   ,   )  # apply function rotr90 elementwise, expand singleton dimensions
rotr90.([A],0:3)  # yields list of rotated matrices:
# [rotr90(A,0), rotr90(A,1), rotr90(A,2), rotr90(A,3)]
sum(                )  # sum

1

# Julia 0.6, 28 bytes

~A=sum(rotr90(A,x)for x=0:3)


Try it online!

Redefines the unary operator ~.
An array comprehension builds a list of 4 rotated matrices, which is then summed.