Volts drop and maximum cable run for a 4 KW solar PV system
AC side.

Figures from BS7671 : 2008
Incoming Voltage single phase 230v
4 KW Solar PV System.
4000 watt / 230 v = 17.39 Amps
Maximum Volts Drop = 1% = 2.3 Volts

Current Carrying Capacity Table 4D2A reference Method B enclosed in conduit.

1.5 mm = 16.5 amps (not acceptable as below the 17.39 amps required)
2.5 mm = 23 amps (maximum length 7.34 meters but not recommended by SMA)
4.0 mm = 30 amps (maximum length 12 meters)
6.0mm = 38 amps (maximum length 18.11 meters)
Volts drop Table
2.5 mm = 18 mV/A/m
4.0 mm = 11 mV/A/m
6.0 mm = 7.3 mV/A/m
Example if we want to use a 2.5mm Cable this is how we work out the volts drop.
Amps x Length x Volts Drop /1000 = 2.3 volts (maximum volts drop)
17.39 x Length x 18 /1000 = 2.3
17.39 x 18 x Length = 1000 x 2.3
313.02 x Length = 2300
2300/313.02 = Length
Length = 7.34 Maximum Length in Meters

This is why we use 4.0mm or even 6.0mm cable from the mains to the inverter.

Using the same working the maximum length for 4.0mm is 12 meters and for 6.0mm is 18.11 meters.

You need a 6.0mm cable running from the mains DB to the inverter to comply to BS 7671.