The possible reason is that the box has got confused because the usual channels you watch and record from are being received from a different transmitter. This can cause the timer to look for the start of the BBC4 programme from that transmitter instead of the one it will be broadcast from. Are other BBC programmes recorded OK.
To ensure that it is not caused by being tuned to more than 1 transmitter action the instructions in http://myhumax.org/forum/topic/week-old-9300t-and-epg-wont-fill-up-for-week#post-7097
Another possibility is that there is some sort of issue caused by other recordings in the schedule. Do you have other timers that record from any channels just prior to the BBC4 timers?