Here is the topology you just described

+-------------+
|             ^
|             |
V             |
M1 --> M2 --> M3
|
|
+----> S1

You would like to slip S1 as a Master into the Replication Ring so that it looks like this:

+---------------------+
|                     ^
|                     |
V                     |
M1 --> S1 --> M2 --> M3

Essentially, you only have to prep S1 to a Master and Point M2 to receive binary log entries from S1.

OK Here we go

STEP 01) Point your application at M3

STEP 02) Prep S1 to be a Master

• set the server_id as a different number from M1, M2, M3
• set log-slave-updates in my.cnf like you did for M1, M2, M3
• enable binary logging on S1 the same way you enabled it on M1, M2, M3
• restart mysql on S1

STEP 03) run STOP SLAVE; on M1

STEP 04) run SHOW SLAVE STATUS\G on S1 and M2

Make sure Seconds_Behind_Master is 0 on S1 and M2

STEP 05) run SHOW MASTER STATUS; on S1 (Record the binary log and position)

STEP 06) run this on M2

STOP SLAVE;
CHANGE MASTER TO master_host='IP Address of S1',
master_post=3306,
master_user='repluser',
master_password='replpass',
master_log_file='XXXX'
master_log_pos=YYYY;
START SLAVE;
SHOW SLAVE STATUS\G

where XXXX is the binary log from STEP 05 and YYYY is position from STEP 05

If the SHOW SLAVE STATUS\G says Yes for Slave_IO_Running and Slave_SQL_Running then you have achieved this:

+---------------------+
                      ^
                      |
                      |
M1 --> S1 --> M2 --> M3

STEP 07) run START SLAVE\G on M1

STEP 08) run SHOW SLAVE STATUS\G on M1, S1, M2, M3

Where Seconds_Behind_Master is 0 on all the servers...

STEP 09) Point you application to other servers as desired

Any questions ???

If none

Give it a Try !!!

Your mission, should you decide to accept this, is to practice this is a Dev/Staging Environment and make sure you trust this algorithm before doing this in Production. In the event your data is caught or killed, the DBA StackExchange and I will disavow any knowledge of your actions.