Update windows install steps for 4.0.0

It omits manual registration step as Windows service.

install/install-by-msi.md

@@ -23,7 +23,7 @@ td-agent.
 
 Please download the `.msi` file from here, and install the software.
 
--   [Download](https://td-agent-package-browser.herokuapp.com/3/windows)
+-   [Download](https://td-agent-package-browser.herokuapp.com/4/windows)
 
 
 ## Step 2: Run td-agent from Command Prompt
@@ -67,19 +67,11 @@ It's working properly if td-agent process outputs the message
 [![](/images/td-agent-windows-prompt.png)](/images/td-agent-windows-prompt.png)
 
 
-## Step 3: Register td-agent to Windows service
+## Step 3: Run td-agent as Windows service
 
-Next, let's register td-agent to Windows service to permanently run as a
-server process. Please execute `Td-agent Command Prompt` again but with
-administrative privilege, and type the two commands below.
-
-```
-> fluentd --reg-winsvc i
-> fluentd --reg-winsvc-fluentdopt '-c C:/opt/td-agent/etc/td-agent/td-agent.conf -o C:/opt/td-agent/td-agent.log'
-```
-
-
-## Step 4: Run td-agent as Windows service
+Since version 4.0.0, Td-agent is registered as Windows service
+permanently by msi installer.
+You can start Td-agent service by manually.
 
 ### Using GUI
 
@@ -111,7 +103,7 @@ The log file will be located at `C:/opt/td-agent/td-agent.log` as we
 specified in Step 3.
 
 
-## Step 5: Install Plugins
+## Step 4: Install Plugins
 
 Open `Td-agent Command Prompt` and use `fluent-gem` command.