I'd go RX 460 2gb. The Athlon II platform, even a quad core, is not up to games like Battlefield 1. So a 460 would give you a whole new level of performance over that 9400, and make previously unplayable games very playable, without requiring you to spend money on a card your CPU can't really use to its potential. It's also not very demanding in power requirements, so your power supply should be able to cope.
A 750 Ti would also be good, but only if it's much less expensive, since the 460 is a bit better and newer in features(like 4k support for video). So if they're close in cost you might as well get the 460.
If that's not inexpensive enough, look for a used card. Just make sure it has 2gb vram, so 1080p is more of an option...