Edexcel M4 is, in my opinion and that of my last class, easier than M3 - provided you have the differential equations background from FP2. I would agree, though, that if you're serious about continuing with a mathematical subject you would be well served by taking FP3. As said above, you need to ask yourself what your purpose here is.
D1 is easy and interesting, but the exam is lengthy and tedious. It rewards accuracy more than intellect, but if accuracy is a strong point for you it's an easy module. I have no experience of S3, so can't comment.