Industry 4.0 (I4.0) is vital to advancing manufacturing capabilities to a new level with highly transparent data for decentralized decision-making and flexible manufacturing technologies for mass customization. However, full embracement of I4.0 technologies requires high investments, whereas the resulting return on investment is difficult to predict. This makes adopting I4.0 particularly challenging for small and medium-sized enterprises (SMEs) as they cannot afford investment failures due to severely constrained resources. Despite various studies on SME-specific adoption barriers and methodical approaches, there is no systematic overview of how existing approaches address which barriers. This would be essential to better understand and help develop targeted approaches for successfully adopting I4.0 in SMEs. Based on a systematic literature review, this paper identifies, categorizes and maps I4.0 adoption barriers and existing methodical approaches. Aside from a socio-technical barrier taxonomy, the mapping identifies groups of well-supported barriers and barriers with only limited or even no dedicated approaches. Thus, this paper contributes by mapping adoption barriers and methodical approaches, highlighting popular research areas and gaps for future research. In the medium term, this will help SMEs systematically plan and execute the adoption of I4.0.