Find the 4th last object in this campaign

My campaigns all follow the same naming convention, i am trying to use a Regex_Extract formula in DataStudio to create a new custom dimension that only displays the 4th last element in the below campaign: "ctatext"

Each element is separated by a _

channel_product_country_medium_brand_offer_campaignname_ctatext_date_objective_0

Closest i have gotten is:

REGEXP_EXTRACT(Session campaign, '(.+){4}(?:[^_]+.)')

but that only returns the 4th character in the string .. can anyone help ?