To determine how many solar panels you need to produce 4000 kWh of electricity per year, you must take various factors into account.

1. Power of the solar panels: The power of a solar panel is expressed in Watt-peak (Wp). This indicates how much power the solar panel can generate under ideal conditions. Standard solar panels generally have a power between 250 Wp and 400 Wp per panel, but this can vary depending on the type and brand.
   
   

2. Solar irradiation: The amount of sunlight that falls on the solar panels varies depending on the location, orientation, and tilt angle of the panels. In sunny areas, solar irradiation can be higher than in areas with fewer sunlight hours.
   
   

3. Efficiency of the solar panels: The efficiency of the solar panels also plays a role. The higher the efficiency, the more electricity they can generate per square meter.
   
   

4. Consumption pattern: Your current electricity consumption is an important factor in determining the number of solar panels you need. The more electricity you consume, the more solar panels are needed to meet that consumption.

To make a rough estimate of the number of solar panels you need, you can use the following formula:

Number of solar panels = Annual electricity need (in kWh) / (Power of one solar panel (in kWh) x Total system efficiency)

The total system efficiency includes the efficiency of the solar panels, cabling, inverter, and possibly other system losses. A realistic estimate for total system efficiency is usually between 70% and 90%.

Keep in mind that this formula provides a rough estimate and it is advisable to consult a professional solar panel installer for a more detailed analysis based on your specific situation and needs.