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:
| Main author | No. of patients | Duration of follow-up years | Linearized rate of infection | Actuarial freedom from infective endocarditis |
| --- | --- | --- | --- | --- |
| Pavie | 675 | 5 |  | 98.2% for all patients, 97.8% for aortic, 99% for mitral, 100% for multiple valve replacements |
| Duncan (A) | 2720 | 6 |  | 97.3% for all patients, 97.4% for aortic valves, 97.6% for mitral valves, 96.3% for multiple valve replacement |
| Ionescu (B) | 1171 | 10 |  | 93.7% for all patients, 94.7% for aortic valves, 97.1% for mitral valves, 89.3% for multiple valve replacement |
| Zhu | 520 | 10 |  | 98% for all patients |
| Revuelta | 239 | 8 | 0.67% per patient year |  |
| Garcia-Bengochea | 248 | 8 | 0.78% per patient year |  |
| Holden (C) | 290 | 6 | 1.1% per patient year |  |
| Bachet (D) | 224 | 6 | 1.6% per patient year |  |
Question:
how many looked at more than 500 patients?
Answer:
4