[Samba] how to set different permission for samba share

[Fatemeh Mehdizadeh]

Hi all,
I'm on FreeBSD11 and I install samba36.
I have some users and groups.
user1, user2 -> group1
user3->group3
user4,user5->group4

I have a share: /extra/samba-test

Access to the share is :

user1->writable
user4->readable
group4->writable
group3->writable

My config file is:

[samba-share]
comment= aa
path= /extra/samba-test
public= no
browsable= yes
writable=no
write list=@group4 @group3 user1

The  question is:
1. How should I say user4 has readonly access to the share?
2. how should I set the permission in unix?

Thank you for your reply and help.