4/8 is actually less than 5/6.
Multiply the 4 in 4/8 by 6 and then multiply the 8 also by 6. You should get 24/48.
Then you should multiply the 5 in 5/6 by 8 and the 6 by 8. You should get 40/48.
4/8 = 24/48 and 5/6 = 4048
Since 40/48 is larger than 24/48 this proves that 5/6 is greater than 4/8.
Hope this helps