I genuinely believe it was quite a favour for Challenge to do it this way. Apart from actually trying to gauge opinions, Series 3, in my opinion, is the one which really - magnets viewers, especially new watchers. It is far longer than S1, which was quite handy, meant more showings to watch, and because it is generally conceived to be the most popular - which was then supported by impressive viewing figures on the repeats, it could be that choice of series which has led to anything else being shown at all.
Mr Child has already said, as I recall, that due to costs incurred, it must really have taken some overwhelming statistics for Challenge to consider S4. Therefore, it could have been the rather illogical starting point that give allowance to any more series showing at all. Let's just hope the trend continues.