SQL & PL/SQL :: Reset Row Number

I have Follwoing Table and data.

CREATE TABLE ABC(DPT_NUM NUMBER,LOT_NUM NUMBER);
Insert into ABC(DPT_NUM, LOT_NUM, SRL_NUM) Values (1, 501, 1);
Insert into ABC(DPT_NUM, LOT_NUM, SRL_NUM) Values (1, 502, 2);
Insert into ABC(DPT_NUM, LOT_NUM, SRL_NUM) Values (1, 509, 3);
Insert into ABC(DPT_NUM, LOT_NUM, SRL_NUM) Values (1, 511, 4);
Insert into ABC(DPT_NUM, LOT_NUM, SRL_NUM) Values (1, 503, 5);

[Code]...

I write Following query.

SELECT DPT_NUM,LOT_NUM,ROW_NUMBER() OVER(PARTITION BY DPT_NUM ORDER BY DPT_NUM) SRL_NUM FROM ABC;

Result is

DPT_NUMLOT_NUMSRL_NUM
15011
15022
15093
15114
15035
15166
15227
15698

[Code]...

But i want to reset rownum after each 4. Mean after serial number 4 , serail number must start from 1. I want to reset serial number after each 4 records (lot_num) against dpt_num;