My function keeps on printing 4.0 as the median instead of 4.5 when I call the list [4, 5, 4, 5]

# x is the users input sorted, use x as parameter
# a is the index of the middle number in the odd version of x
# b is the 1st of the indexs in the even version of x
# d is the result of the 2 middle numbers
# being added together in the even version of x

def median(lst):

    x = sorted(lst)

    if len(x) % 2 != 0:
        # If the users input is odd
        a = len(x) // 2
        return(x[a])

    elif len(x) % 2 == 0:
        # If the users input is even
        b = len(x) // 2 - 1
        c = len(x) // 2
        d = (x[b] + x[c])
        ev_median = (d / 2)
        return(float(ev_median))

print(median([4, 5, 4, 5]))