[tex]\dfrac{18}4=4+\dfrac24[/tex]
which means 18 can be divided by a higher power of 4 to get a quotient smaller than 4. Indeed,
[tex]\dfrac{18}{4^2}=\dfrac{18}{16}=1+\dfrac24[/tex]
Now divide the remainder by 4 gives
[tex]\dfrac24=0+\dfrac24[/tex]
and so
[tex]18_{10}=102_4[/tex]
Now the next four numbers are easy to find.
[tex]19_{10}=103_4[/tex]
[tex]20_{10}=110_4[/tex]
[tex]21_{10}=111_4[/tex]
[tex]22_{10}=112_4[/tex]
[tex]23_{10}=113_4[/tex]
