<div class="profile-pic pic{{id}}">{{ fields.field_picture.content }}</div>

{% set id = 0 %}

Picture 1 ==> class = profile-pic pic0
Picture 2 ==> class = profile-pic pic0
Picture 3 ==> class = profile-pic pic0
Picture 4 ==> class = profile-pic pic0

Picture 1 ==> class = profile-pic pic0
Picture 2 ==> class = profile-pic pic1
Picture 3 ==> class = profile-pic pic2
Picture 4 ==> class = profile-pic pic3

{% set id = id + 1 %}

{% for field in fields -%}
<div class="profile-pic pic{{id}}">{{ fields.field_picture.content }}</div>
{%- endfor %}