Four weeks ago, I had a meeting over Skype with my potential PhD supervisor for an open PhD position in his group. He initially expressed interest in my work and asked me to arrange for two recommendation letters in order to know more about me and then later, we discussed about my current/past research works, my future plans and intricacies of his work for about 45 minutes over Skype. He had also specified that both of my letters were good and will discuss with his boss (his group leader) regarding my academic background and research experience. Finally he told that he had applied for a grant and will let me know once he gets to know about the outcome. Overall, the meeting went quite well. Since then, I haven't heard back from him. He also hasn't responded to a couple of my reminder emails. Should I be optimistic about this?
Call him or his assistant. Or drop by during his duty hours or after his lectures.
No, you shouldn't be optimistic, you should continue applying for other positions.