How do the plates over there work ?only having 4 numbers and all.

The plates work like this:

1. The kanji is for the prefecture
2. The numbers at top right echo the prefecture* (5 is Niigata) + a sequence (extra digit circa 1969, third digit on newer cars, It seemed to me as if 2004 cars had them, earlier did not, at least in Niigata Prefecture)
3. There are more than four digits in the main area -- there is also a small hirigana character at the left.

In-depth details: Tigerdude's License Plate System of Japan.

*I asked about the "class" system Tigerdude documents, but was told it is based on year and prefecture rather than engine size. So I'm not really sure.

K-cars have yellow plates. Big cars like our Datsun 1200s would have a normal white plate.

What I know is written.
1 Nigata is a meaning of the prefecture name.
2 58 is a meaning of the compact car.
(651cc/2000cc)
The large-sized car in case of 3.
3 The hiragana is private or different by the rent-a-car.
4 The favorite one can be chosen to be a figure.

How is Datsun or Datsun 1200 written in Japanese?

ダットサン

Is from google translate for datsun

Yes indeed. It is a phonetic spelling of "dattosan"