[рони]

firsmember,
а так?

jQuery('.slider-prm-rab-box-block').each((i, el) => {
jQuery('.slider-for', el).attr('id', `fff${i}`);
jQuery('.slider-nav', el).attr('id', `nnn${i}`);
jQuery('.slider-for', el).slick({
  slidesToShow: 1,
  slidesToScroll: 1,
  arrows: true,
  fade: true,
  asNavFor: `#nnn${i}`
});
jQuery('.slider-nav', el).slick({
  slidesToShow: 4,
  slidesToScroll: 1,
  asNavFor: `#fff${i}`,
  vertical: true,
  verticalSwiping: true,
  dots: false,
  focusOnSelect: true
});
})