My question is why there are only four khalifas and not a fifth one? Furthermore I would like to ask how I can best explain to an Ahmadi about the true religion.
That the rightly guided caliphs are only four is a judgment of the believers. They think that only four of them followed the Prophet (sws) in the truest way. If someone today follows the Prophet (sws) in that sense he too would be considered khalifa e rashid. Many people hold that Umar b Abdul Aziz, an Umayyad caliph, was also also a rashid khalifah.
As for interaction with the Ahmadis I believe it is not the duty of every believer to be able to convince the Ahmadis or the followers of any other faith for that matter. It is the duty of the scholars to invite others to the true spirit of Islam with arguments. My duty is only to keep my faith intact and make sure tha the people in my influence are exhorted upon the right and fobidden from wrong.
May God help us tread path of his liking.