The fourth declension was one of the less common inflection pattern for Latin nouns, and the vast majority of fourth declension nouns are masculine nouns ending in the deverbal abstract noun suffix -tus (-sus).
I'm wondering why the word porticus 'colonnade, piazza, portico', which appears to be a derivative of porta 'gate', ended up being part of the tiny group of feminine nouns in Latin with the fourth-declension inflection pattern. What is the formation of this word? Latin has a derivational suffix -icus that forms first/second adjectives ending in -icus, -ica, -icum; if this is present in porticus, I would expect the form and gender to instead be porticus, -ī m. or portica, -ae f. (or porticum, -ī n.).
Lewis and Short mentions that a second-declension accusative plural PORTICOS is also attested, although I don't know how significant this is to the etymology of this specific word, considering my understanding is that many other fourth declension nouns showed some variation with second-declension forms (per Alex. B's answer here).