There are several ways to accomplish what you're saying, unfortunately the question does not provide enough information to delimit the scope of the answers you could get and thus the one you really need.
%option1
for i = 1:3
vec1 = rand(4,1)
vec2 = rand(4,1)
end
var = [vec1;vec2]
%option 2
var = [];
for i = 1:3
var = [var;rand(4,1)]
end
...and so on, depending on the data structure you have.
