I used to have a '69 model.  It was a SR but PS were also made. They were Deere's first turbo ed tractor...and they got it right.  They were made in '69 and '70.  The latest one's were inter cooled.  They had dual single stage air filters with an exhaust aspirated honey-comb pre cleaner that was very effective.  According to JR Hobbs, they had air cleaner failures (fires) and they were changed to the 4620 design with a single primary/secondary air filter system.  Mine was still the original design and I had no trouble with it.  As I said, the pre-cleaner worked really good.  Deere developed the 20.8 38 tire for the 4520.  I still have an original brochure from when they were new which explains that fact.   Mine had between 9000 and 9500 hr on it when I sold it (wish I still had it).  The engine was all original (except I rolled a new set of bearings in it at about 7200 hr for good measure...friend had a 4620 which wrecked a crank at the time) and it still started good.  The starter bushings were worn which caused it to turn rather slow.  You would turn the key and the starter would lumber along turning the engine very slow and she'd always light up and go.  I had the starter gone through (didn't realize at the time how bad it needed work) and it was surprising how much faster it spun.  The replies that stated they were a lumbering heavyweight like a 5020 are just plain wrong.  The 4520 was very agile, not like a 5020 at all.  4520's are physically much larger than a 4020 but relatively speaking they are just as maneuverable.  (I have a 4020 also)  It was a very comfortable tractor to drive.  They were rated at 122 hp.  I also had a 4250 at the same time.  It was rated at 120 hp.  Those two tractors were very similar in pulling performance, weight (I had them both set up with radial 20.8 38 duals, recommended ballast and front/rear weight split)  and horsepower-hour/gallon by the Nebraska tests.  The 42 could out-work the 45 at times because of having twice the gear selection.  The 45 and 42 were equal in maneuverability.  The 45 was indeed a heavily designed tractor since it weighed the same without a cab as the 42 did with a cab.  There was a 4520 near here vor many years sporting an 8V-71 Detroit conversion and the transmission and rear end lived! 

Can you tell I miss that tractor?