Counting models satisfying a boolean formula
For any fixed $k$, a $k$-CNF with at most four clauses has at most $4k$ variables. So you can count the satisfying assigments with

count = 0
j = number of variables
for v1 = 0 to 1 do
    for v2 = 0 to 1 do
        ...
        for vj = 0 to 1 do
            if formula_value(phi, v1, ..., vj) == true
                count = count + 1


This runs in time $\Theta(2^j) = O(2^k) = \Theta(1)$, since $k$&nbsp;is fixed.