Is a four-rak'ah prayer after Dhuhr valid if one intends to shorten Asr?

It is not permissible to avail oneself of the concessions for travel until one has left the city's populated area. The mere intention to travel does not permit taking advantage of its concessions. The fundamental principle for combining prayers is succession (muwalat), based on the action of the Prophet (peace be upon him). If the combination is at the time of the first prayer, there must be succession between the two prayers; if a long time separates them, the combination becomes void. If the combination is at the time of the last prayer, a separation between them does not harm, because once the first prayer is performed, the second will be prayed within its own time.