Hi,
I can offer a few thoughts based on my experience of making hay and owning a TED 20 (among others) a various mowers over the years and a baler.
Your TED20 has a few limitations in power and size, but for only 4 acres you will be able to have some fun and it is very satisfying.
Mowing- The mower ferguson produced to work with the TED20 was the finger bar mower, there was an early version which is all floppy and is a nightmare - steer clear. The slightly later version is i think is the 735 a finger bar mower but distinguishable by its heavy frame at the front. Farmers dont use these any more, and they can be picked up reasonably on ebay- though it could be a risk on reliability, and they are easily damaged. You still can buy the blades not sure about anything else though. You will need an original style T20 top link with rack and chain as the fergy has no position control on its hydraulics.
The 5'6" drum mowers used by farmers for years which followed this are simply too big for the 20, dont be tempted.
Turner- Simple- buy a PZ haybob an early one (i think designated the 270) the later 300 (distinguishable by fatter tyres) would be too big for your T20. This will do your turning and rowing up. Your t20 will be a bit light on the front end when lifting it, if the hydraulics are tierd it may struggle, but if your just in one field attached to your place you can get away without actually having to lift it.
Baler-- Your t20 will be out of its depth here, balers in the days of T20s had their own engines mounted on them, so all the T20 was doing was towing it around.
My suggestion would be as follows:
For year one:
You only mow the field once, you only bale the field once... so get your local friendly farmer contractor/ vintage enthusiast to do it.
You have to turn the hay 2-3 times a day for about a week, then row up., When the weather window arrives everyone is using their turners and dont want to lend them (i speak from bitter experience)--so buy a haybob, with £600 in your pocket you should easily find a good one. You can get some one to mow then spend the next week turning it yourself.
Buy a trailer for the fergy, you want a 3ton one, then you can start carting as soon as the baler man gets going.
Once youve done a year (or 2) youll have experience and can decide on further investment, mowers balers a bigger tractor etc. ---If you change your mind youll always get your money back on the haybob, and trailers are simply invaluable all year round.
As you have found out you need an adaptor for the PTO- or you can easily upgrade to the bigger size by replacing the shaft in the tractor, its a simple job and part readily available. Worth considering if you do a lot of PTO work, in my experience the adaptors wear quite quickly.
Hope my thoughts are of help any questions dont hesitate. I got a baler and a mower first and soon realised its the turner you really need !
stu