Anybody can check my Markov Chain (scheme inside)?

[Gudsaf]

I draw scheme for 1oo2D or 2oo4 (hot reserve):

For that scheme i determined the following events:

• s0-system works
• s1-denied one of the nodes A1 or B1
• s2-denied one of the nodes C1 or D1
• s3-failure of the system

I obtain that Markov Chain:

Can anybody check that chain - is it true?