A training organisation at my local airfield (Blackbushe) went into liquidation a few years ago and a lot of people lost an awful lot of money. Even where a company goes into solvent liquidation it can be difficult and time consuming to recover what is owed.
Would you pay your local 2nd hand car dealer £4000+ up front, for a car you will not take delivery of until November? The security of your money is about the same with the average flying club.
If you must pay up-front, pay only some of the money and do it with a credit card.
That said, there are 101 reasons or more why you might not finish the course, or might have to postpone it. I would definitely recommend paying as you go.
HH