Xiaomi loves releasing budget phones in series with small variations in their hardware configuration. It's like the company can't settle for just one configuration and wants to cover every little niche you may think of. Whether it's a fingerprint scanner, a better camera or a camera flash, bigger storage or RAM, newer chipset, or even higher screen resolution - there will be a Redmi out there meant to answer your specific requirements.
Source: http://www.gsmarena.com/xiaomi_redmi_3s_prime_vs_redmi_4_prime-review-1540.php
