With rods 4 and 6 it is possible to make a solution as you could use the subtraction 6-4 to create 2.
It is not possible to make the number 8. This can be proven by trial and error by trying each possible calculation and checking if it can make 8.
3+4=7
3+6=9
4+6=10
6-3=3
6-4=2
3+4-6=1
6+4-3=7
6+3-4=5
These are all the possible combinations of the rod that can be made. As you can see it is clearly not possible to create a value of 8.
To create a value of 1-8 or 1-9, we must keep the same principle that there are two numbers that are 1 apart with a third number that has a higher value than the pair of numbers and to help fill in the missing numbers that the pain cannot create. An example of this would be 4,5 and 7 this combination of numbers would be capable of:
5-4=1
7-5=2
7-4=3
4
5
7+4-5=6
7
With this combination you would have been able to achieve a value of 1-7. Now if we keep trying with this method a possible answer for the values 1-9 would be 2,3 and 6. With this set we can make all the values from 1-9.
3-1=1
2
3
6-2=4
2+3=5
6
6+3-2=7
6+2=8
6+3=9
For the values of 1-8 the set that I have found was 1,2 and 5
1
2
1+2=3
5-1=4
5
5+1=6
5+2=7
5+1+2=9
In conclusion following the pattern that there should be a pair of numbers and then another number larger that the pair and experiment with that it is possible to create the values of the numbers 1-9 and 1-9.