These values apply to the expected input and output levels. The levels of +4dBm and -10dBV are the 'nominal' levels, not the maximum or minimum, but used to differentiate between equipment for 'consumer' applications which generally have lower level, high inpedance inputs and outputs (the -10dBV level) and equipment for 'professional' applications, which have higher level low impedance inputs and outputs (the +4dBm level).

Both are voltage levels, and as you will see from the table below that the +4dBm equipment will produce up to 4x the level of the -10dBV equipment

If you connect equipment with different nominal levels, then yu may experience distortion, particularly if a high impedance output is connected to a low impedance input, as the output may not be able to deliver the necessary voltage. Generally, low impedance outputs will connect to high impedance inputs, although you will have to be careful with the level output.