Kevin,

really good brute force solver, you've made!

Are you sure

    return solve(a, b, c+1, 0) if d==3
    return solve(a, b+1, 0, d) if c==3 
    return solve(a+1, 0, c, d) if b==3  

shouldn't be

    return solve(a, b, c+1, 0) if d==3
    return solve(a, b+1, 0, 0) if c==3 
    return solve(a+1, 0, 0, 0) if b==3  

?

(I rewrote it in Ruby)

regards
Christer