call drop on the TCH channels and SDCCH congestion are different issues, therefore adding SDCCH capacity won’t help.
are you having SDCCH drops or TCH drops ? I guess it is TCH drops (= call drops)
The last trx of the cell is usually the one that carries the highest amount of traffic. Because you lock the TRX #4, the traffic is now being mostly carried by TRX #3. Therefore, check interferences…
You says “there are more intracell HO”. Well, maybe it is because the TRX3 is interfered, therefore most of the traffic is interfered. It leads to a higher amount of interference HO than before. For instance, before locking, there was 80% of calls on TRX4, with good quality. And 20% on TRX3, with poor quality due to interference.
After locking : 0% on trx4, 80% on trx3 and 20% on trx2. Maybe there is interference on trx3, or possibly trx2. You need to check indicators per TRX. I can’t tell you where you can find them in Siemens because i work with a different vendor, but you’re supposed to know 😉
To conclude, i’d say you really need to investigate more into details your problem : more indicators (tch mean duration, per trx, tch assignment failure, sdcch drops), more drive-tests, and try to lock other TRX instead of TRX4.
And finally, check your frequencies !! are you using frequency hopping ?