Shimano Tiagra ST-4603 Name Plate & Fixing Screw - Left - Y6UP98020

I recommend this product

If you've got tiagra 4600 shifters, you'll know that these little plastic covers are both fragile and vulnerable. If you have a falling-off, these will probably get damaged, leaving the rest of the shifters OK.

I'd only give the parts themselves 3 stars, cos they're a rubbish design and overpriced. However, it's a lot better than buying new shifters. SJS is the only place online I can find this sort of useful spares, so I give them 5 stars for that alone.