A Study in Scarlet is wonderful right up until the point where there's an entirely separate story wedged into the middle of it. I understand the relevance of having the perspective of the criminals, but I think ACD got a little carried away. I've never read this before, however, so to see the first ever meeting of Sherlock Holmes and John Watson was enjoyable, and seeing how they grow to each other.
A Sign of the Four, on the other hand, is definitely my preferred of the two. Maybe I pay too much attention to and grow easily annoyed by plot holes, but there were clear mistakes in continuing from the previous book to this, not to mention the odd error throughout this story. But, as we only moved away from Holmes and Watson at the very end to discover the missing details of Sherlock's deductions, it was enjoyable, easy to get wrapped up in, and less complicated to follow.
I have to admit, though, that I think picturing Holmes and Watson as Sherlock and John from 'Sherlock' helped get me through both of these stories, and picking out the bits that the modern day show updates and fits in was interesting to me. Over all, I thoroughly enjoyed both novels, despite the odd moments of struggling to get through parts, and I'll definitely be reading both again in future.