Python 3, 64 55 51 bytes

-9 bytes thanks to @ovs

lambda s:choice((s*2)[1:]+s[1])
from random import*

Try it online!

##Explanation

random.choice() gets a random character of the String, while (s*2)[1:]+s[1] creates BCDABCDB for an input of ABCD, which has 1/8 As, 2/8 Cs, 2/8 Ds and 3/8 Bs.

Python 3, 64 55 51 bytes

-9 bytes thanks to @ovs

lambda s:choice((s*2)[1:]+s[1])
from random import*

Try it online!

Explanation

random.choice() gets a random character of the String, while (s*2)[1:]+s[1] creates BCDABCDB for an input of ABCD, which has 1/8 As, 2/8 Cs, 2/8 Ds and 3/8 Bs.

Python 3, 64 55 51 bytes

-9 bytes thanks to @ovs

lambda s:choice((s*2)[1:]+s[1])
from random import*

Try it online!

Python 3, 64 55 51 bytes

-9 bytes thanks to @ovs

lambda s:choice((s*2)[1:]+s[1])
from random import*

Try it online!

##Explanation

random.choice() gets a random character of the String, while (s*2)[1:]+s[1] creates BCDABCDB for an input of ABCD, which has 1/8 As, 2/8 Cs, 2/8 Ds and 3/8 Bs.

Python 3, 55 bytes

-9 bytes thanks to @ovs

lambda s:choice((s[0]+s[1:]*3)[:8])
from random import*

Try it online!

#Explanation

random.choice() picks a random character of the String, (s[0]+s[1:]*3)[:8] generates a new string with the following structure: ABCDBCDB, for an input of ABCD.

Python 3, 64 55 51 bytes

-9 bytes thanks to @ovs

lambda s:choice((s*2)[1:]+s[1])
from random import*

Try it online!