Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Year | Entrant | Chassis | Engine | Points |
| --- | --- | --- | --- | --- |
| 1979.0 | Team Ensign | Ensign N179 | Cosworth V8 | 0.0 |
| 1980.0 | Team ATS | ATS D3 | Cosworth V8 | 0.0 |
| 1980.0 | Team ATS | ATS D4 | Cosworth V8 | 0.0 |
| 1981.0 | Ensign Racing | Ensign N180B | Cosworth V8 | 4.0 |
| 1981.0 | Theodore Racing Team | Theodore TY01 | Cosworth V8 | 4.0 |
| 1982.0 | Arrows Racing Team | Arrows A4 | Cosworth V8 | 3.0 |
| 1982.0 | Arrows Racing Team | Arrows A5 | Cosworth V8 | 3.0 |
| 1983.0 | Arrows Racing Team | Arrows A6 | Cosworth V8 | 4.0 |
| 1984.0 | Barclay Nordica Arrows BMW | Arrows A6 | Cosworth V8 | 1.0 |
| 1984.0 | Barclay Nordica Arrows BMW | Arrows A7 | BMW Str-4 t/c | 1.0 |
| 1985.0 | Motor Racing Developments Ltd. | Brabham BT54 | BMW Str-4 t/c | 5.0 |
| 1986.0 | Barclay Arrows BMW | Arrows A8 | BMW Str-4 t/c | 0.0 |
Question:
How many years was ensign n180b a Chassis with 4 points?
Answer:
1981.0