I wouldn't really recommend adding a GPU if you're on a tight budget, since most of the common functionalities of Photoshop and Lightroom don't utilize the processing power of the GPU (OpenCL). It's only on some specialized filters and effects that OpenCL is really used, and the HD4400 is enough for that (I mean, what is a few minutes, at most, difference gonna make with applying a complex filter?). A GPU would be very useful though for Premiere Pro, to enable the Mercury Playback Engine (for CS6 and older, Nvidia's CUDA is required; for CC, both AMD and Nvidia cards will increase performance).
On the other hand, if you have spare cash to spend on a GPU, for purely photo-editing purposes, it would be wiser to upgrade to a Haswell i5...