The assessment of the digital level is considered pivotal, for manufacturing companies including Small- and Medium-Sized Enterprises (SME), at the outset of implementing Industry 4.0 solutions in their digital strategies. Several self-assessment tools exist measuring the digital readiness and maturity, returning the overall digital level of the companies. On the one hand, the application of such tools proved to be effective, as it enables companies to systematically reflect about their digital level, strengths and weaknesses as well as opportunities and challenges to consider prior to the implementation of Industry 4.0. On the other hand, particularly for SME facing higher challenges compared to large companies in the management of digital transformation, relying exclusively on the knowledge of their overall digital level may offer limited elements to strategically orient decision-making process in this field. The present study presents the methodology used to develop, within the framework of a self-assessment tailored to the requirements of SME, a set of Key Readiness Indicators (KRI), deepening the interpretation of the overall digital level of the companies in specific areas of interventions. The KRI focus on the digital readiness of companies in terms of strategy, technological requirements, awareness about digital trends, and competences of employees, to offer complementary information to ease the definition of strategies for technological implementation in SME. Besides the methodological approach employed, the study shows the distribution of KRI within a sample of manufacturing companies located in the Marche region (Italy) taking part to the proposed self-assessment. The emerging dynamics of the distribution of KRI according to varying company characteristics such as size, turnover and overall digital level will be presented and discussed, together with the main implications related to the use of KRI for strategic planning of Industry 4.0 in manufacturing companies.