'नमः' इति योगे विभक्तिः भवति। 'नमः' इति पदस्य योगे -
English: चतुर्थी is correct vibhakti vachana form of given shabda as per declension rules of Sanskrit grammar. Hindi: चतुर्थी सही विभक्ति वचन रूप है जो संस्कृत शब्दरूप नियमों के अनुसार बनता है।
Ref: NCERT Sanskrit Class IX Vibhakti explains prathama to saptami vibhakti karaka relations shabdarupa examples for students clearly.