Reliability of 3oo4 system

[ADALGLISH]

If I have a 3oo4 dissimilar configuration, then where reliability of the equipments are R1, R2, R3 and R4 what is total system Reliability, Rs.

Intuitively I thought

Rs = R1R2R3R4 + (1-R1)R2R3R4 + R1(1-R2)R3R4 + R1R2(1-R3)R4 + R1R2R3(1-R4)

Is this correct ?

Cheers,
al.

[Pantelis]

RS=R1R2R3+R1R2R4+R1R3R4+R2R3R4-3(R1R2R3R4)

[dvaidr]

I've come across this a lot lately. 3oo4 and 1oo2 etc.

What does it mean?

Thanks.