I am finding it hard to categorise my current research study as there are 4 participants and I am using an arts-based method, so I can't decide which method it is?
A focus group is a data collection technique, where data can be selected to use for a multiple case study. Please also find some links that provide some more useful information:
I believe that FGD has been described sufficiently. As for Multiple Case Study, I would suggest reading Yin's 1994 Case Study Research, there you'll find different types of Multiple Case Study e.g. multiple embedded case studies.
Multiple case studies are based on replication logic; either theoretical or literal where as focus group is one type of qualitative interview, which discuss among small group of participants around a special field to reveal opinions and information for further course of action.
A focus group is a technique to collect information, where you meet with four or more people in one or more groups, asking questions, letting them answer, letting them to respond to other's answers, and stimulating everyone to participate and share their views in the topic on which they are interviewed.
A multiple case study is a research design originated in historical-hermeneutic orientation, where you analize the same phenomenon in two or more cases, showing how the phenomenon in presented in each of them (sometimes compraing them, but it is not needed). Those case studies can use many techniques to collect and to analyze information.
Multiple case studies, has discretion information about particular case , imparting special information for reader, is a qualitative study but subjectivity is more while in focus group, also a qualitative study but more objectives and less subjective.
Essentially, 'Multiple Case Study' and 'Focus Group' are exactly 'what it says on the tin';
In a Focus Group,' some people come together---guided by a researcher-facilitator---to 'focus' on a topic; the outcome of the FG is summed up by the facilitator as some kind of collectively agreed on perspective regarding the topic.
Whereas Multiple Case Study is a study of several 'cases' of...whatever.