ENB inform the tm mode to UE through rrc Connection setup or rrc connection reconfiguration message. If eNB has informed tm mode 3/4 to UE then eNB can switch tm mode from 3 to 2 or 4 to 2 without rrc signalling. ENB can schedule 2 code word in tm 3 using DCI 2a or in tm4 using DCI 2. It can switch to tm 2 tx div by scheduling DCI 1a and will schedule 1code word to UE.
Switching between different mode depends on various situations... Suppose eNB has sufficient data to schedule in DL for tm 3/4 and UE also reporting RI 2, channel condition also good then eNB can use tm3/4 to schedule 2 code word.
If enb does not have enough DL data or UE reporting RI 1 and channel condition also not good the eNB can schedule DL data using DCI 1a in tm 2 .