Not sure why you say it is off. filter() convolves the data with the 4-point moving average filter in a causal manner.
So for the first element you get 77/4 because x(0-k) is only valid when k=0, as soon as k increments to 1, you have x(-1) which is 0. There is no element before the starting element, x(0) (not using MATLAB 1-based indexing here)
For the 2nd element you get (x(1-0)+x(1-1))/4 = (77+76)/4
finally the 4th element gives you 75.75
Using zero-based indexing, filter is outputting the following:
Let h(n) = 1/4 n = 0,1,2,3,4
y(n) = \sum_{k=0}^3 h(k) x(n-k)
start plugging in values for n of n = 0, 1,2, 3 and you'll see why you get the output you observe.
