There are one million awesome things you can do with small router and an OpenWRT firmware on it. I have been surprised how mature, easy to use and community driven it has become. The WR703N is an awsome device and I have probably never seen such a small device (smaller than a smartphone) with so many features, an USB, Ethernet, Wifi and Linux running for just under 25$.
However 4 Mb is very few, even to run OpenWrt, and you quickly run into flash memory limits. But you can easily work around this limitation using ExtRoot, which allows you to include external USB devices or even network mounts. However the collection of instructions on how to setup ExtRoot makes it look more complicated than it is, and it can be (almost) performed completely by LuCi, OpenWRTs GUI interface in the latest stable release (Breaking Barrier 14.07).