Having been offered a position myself this year, and meeting many people who have also successfully completed the process, I can confirm that it is no harder to get into a big 4 firm without an accounting related degree or some form of work experience. I have to admit that when I considered applying the same thoughts crossed my mind (I have a biology background with no prior experience), however throughout the process it became clear the big 4 firms place greater emphasis on your talents, general experience, and potential, rather than specific industry knowledge. This is predominantly due to the confidence they have in their own training programs, but also because they want to have a diverse range of experience within their firms. In fact, at a recent event I attended for new starters I met only one business related graduate (albeit I didn’t meet everyone in the room), with the rest made up of people from very unrelated backgrounds (history, law, english, phycology, maths, physics, etc). Interestingly, not one person I spoke to had done an internship or any other form of work experience before applying, so it’s definitely not a prerequisite.
Although it’s clear past experience is not necessary, my general advice would still be to ensure you have a very good idea what the job will actually entail by looking online and, if possible, speaking to people from the firms themselves (they do networking events, branch visits, online chats, etc). I was repeatedly quizzed on my understanding of the role I would be taking on and was, on multiple occasions, asked what I thought would be the biggest negatives of the job. In addition, ensure you have good examples from your past where you have displayed leadership, teamwork, time management, decision making, etc. These certainly don’t have to be business related, but it is important that they fit into the STAR answer format and you can answer follow up questions relating to them.
Hope this helps and good luck.