box% ghci
GHCi, version 8.10.2: https://www.haskell.org/ghc/  :? for help
Prelude> terms = let digs = [1..5] in [a*1000 + b*100 + c*10 + d*1 | a <- digs, b <- digs, c <- digs, d <- digs, a/=b, a/=c, a/=d, b/=c, b/=d, c/=d]
Prelude> terms
[1234,1235,1243,1245,1253,1254,1324,1325,1342,1345,1352,1354,1423,1425,1432,1435,1452,1453,1523,1524,1532,1534,1542,1543,2134,2135,2143,2145,2153,2154,2314,2315,2341,2345,2351,2354,2413,2415,2431,2435,2451,2453,2513,2514,2531,2534,2541,2543,3124,3125,3142,3145,3152,3154,3214,3215,3241,3245,3251,3254,3412,3415,3421,3425,3451,3452,3512,3514,3521,3524,3541,3542,4123,4125,4132,4135,4152,4153,4213,4215,4231,4235,4251,4253,4312,4315,4321,4325,4351,4352,4512,4513,4521,4523,4531,4532,5123,5124,5132,5134,5142,5143,5213,5214,5231,5234,5241,5243,5312,5314,5321,5324,5341,5342,5412,5413,5421,5423,5431,5432]
Prelude> length terms
120
Prelude> sum terms
399960
Prelude>