Always had forward facing seats, so can't comment on their use in older children. It really depends on doing your own risk analysis. What type of driving do you do? What type of car do you have/whats the car safety rating? etc etc.
Only you can decide what's best for your child and I would think the chances of being in any accident are fairly slim, the chances of any accident you have resulting in an injury for any passenger are even smaller, the chances of a good quality, properly fitted car seat not protecting your child are even smaller. There are possibly some accidents that could happen that it would be better to be in a rear facing seat, but the chances of that must be tiny. For me, it wouldn't be so much the cost, but the size of my child and how practical it would be to try to get them in a rear facing seat.
Just checked my sons' charts and they were 3 foot 9 at age 4. Where do they put their legs? Eldest also suffered from car sickness so was violently ill when facing backwards so it wouldn't have worked for us.