Using QGIS OTB plugin

Does currently not work in OTB plugin for QGIS 3.4.

In the search engine of the Processing Toolbox, type PCA and select DimensionalityReduction(pca) under Image Filtering of the Orfeo Toolbox.
Under the Parameter tab, select Multispectral band file as the input layer.
Select pca under the Algorithm tab.
Enter 4 as the number of components.
Specify the directory to save the output file.
Untick output file after running algorithm (inverse output image) option.
Click on Run to execute the algorithm.

Using OTB standalone

Type into the search box of the Windows taskbar: mapla.bat. Click on mapla.bat to open Monteverdi Application Launcher.
In the search engine of the Processing Toolbox, type Dimension and double click Dimensionality Reduction.