PL/SQL Loop Types - Variables Incrementing Together

set server output on;

DECLARE
num number(1) :=1;
num2 number(1) := 1;
BEGIN
WHILE num < 5 LOOP
[code]...........

the output for this loop is:
1+1=2
2+2=4
3+3=6
4+4=8

this is what I wanted. Both variables are incrementing together. However, when I try to use a FOR loop to do the same thing:

set serveroutput on;

DECLARE
num number(1) :=1;
num2 number(1) := 1;
BEGIN
FOR i in 1..4 LOOP
[code].......

I get the result:

1+1=2
1+2=3
1+3=4
1+4=5
2+5=7
2+6=8
2+7=9
2+8=10
3+9=12
DECLARE
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: number precision too large
ORA-06512: at line 8

I understand the output means the inner loop is running its course before the outer loop is started and then continuing on to exceed its limit, but I don't understand why this is or how to make it so I get the same results as the WHILE loops.