+4dB and -10dB are incomplete designations. It should be +4dBu and -10dBV. The letter after the dB designates the reference. dB by itself is only a ratio. In this case dBu indicates a reference of 0.775 Vrms while dBV indicates a reference of 1.0 Vrms. With this information we have: +4dBu = 1.228 Vrms -10dBV = 0.316 Vrms 20*log(1.228/0.316) = 11.79 dB difference between those two standard levels. Single ended is usually associated with -10 dBV levels. Balanced is usually associated with +4 dBu levels. But not always.