a(n) = Sum_{k=1..n} binomial(k+3,3) * floor(n/k).

Terms

a(0) =4a(1) =18a(2) =42a(3) =91a(4) =151a(5) =269a(6) =393a(7) =607a(8) =851a(9) =1207a(10) =1575a(11) =2183a(12) =2747a(13) =3561a(14) =4457a(15) =5640a(16) =6784a(17) =8452a(18) =9996a(19) =12158a(20) =14326a(21) =17004a(22) =19608a(23) =23306a(24) =26642a(25) =30870a(26) =35174a(27) =40518a(28) =45482a(29) =52214