How Much Volts Is +4dBu?

To get this answer, we first find what 0dBm is, which is defined as 1mW into 600R for audio.

P = VI and I = V/R so P = V^2/R.
PR = V^2
V = sqrt(PR) = sqrt(0.001*600) = 0.775Vrms.

The ratio of +4dB (when applied to voltage) = 10^(4/20)
so +4dBu = 0.775 * 10^(4/20) = 1.228Vrms = 1.736Vpeak
(the u in dBu means unterminated).