Mobile operating systems, with few exceptions (see Android), can only be upgraded if the device manufacturer offers the update for the specific model, which is a key difference between the Windows Mobile and Windows distribution channels. In this case, HP is not currently offering upgrades for the hx4700, nor will it in the future, so there is no legal upgrade option for you aside from purchasing a new PDA/smartphone.
My iPAQ hx4700 runs Windows Modil 2003 Second Edition.
Is it possible to upgrade to a newer version of windows Mobil ? How ? Thanks.