When they say you can take a car seat in addition to your normal baggage allowance...

[beansmum]

do they mean any car seat, or just baby ones? Would my 4yo's seat count?

We took our 4yo car seat with us when we flew with Monarch at no extra charge.

I woul dring them to check but when we went to Florida it was any carseat. We wrapped it at the Airport in binbags and gaffer tape so it was a bit protected to.

As far as I am aware, it is any car seat as its law for children to be restrained in car in most countrys and as its a safety thing it would be v bad PR for companies to include that within allowance as significant number of people would choose not to take them!
If in doubt, give the airline a call.

any car seat
we have taken bigger seats, boosters etc

Just something that might be of use, we have just been to NZ and took our carseat, bought this from Amazon to transport it in the hold:
www.amazon.co.uk/exec/obidos/ASIN/B000EDL81I
In the photo it looks like it'll only fit the taller, narrower seats for toddlers, but we had our 5mo's carseat in there with room to spare.

the trick with airline is to say that your buggy breaks into 2 parts - my carseat clips on to the top of the bugaboo so i can get away with not having to fork out for the car seat - it depends on the buggy system you have - if the flight isnt busy most airlines will allow you to bring the car seat on to the plane for babs to sit in - not always a good idea because it is such a s-c-h-l-e-p from the gate to the carousel..sometimes its better to just check it on!