yes : the RLT works for SDCCH as well as for TCH. And it works in the same way, with the same RLT value = 20.
You said tht SDCCH phase lasts only 4s. I agree. But if the radio link is poor, then the phase will last much longer. For ex. the MSC sends the AUTH REQ to the MS. If the MS does not receive this message due to poor radio conditions, then the MSC will retransmit it. Therefore, the duration increases.
At the same time, the MS will decrease the S counter if the SACCH is not received.
What I’m trying to say is that the SDCCH phase is “stalled” when the radio is bad. The S counter has plenty of time to reach “zero”.