The design of effective trust and reputation mechanisms for personal learning environments (PLEs) is believed to be a promising research direction. In this paper, we propose a 4-dimensional trust model that complies with the specific requirements of PLEs. Trust is explored in four dimensions: trustor, trustee, context and visibility. The importance of these four dimensions is investigated through a number of scenarios. The model is implemented in a PLE platform named Graaasp. Preliminary evaluation of usefulness is conducted through a user study and some interesting findings are discussed in the end.