How to use multiple csv files in one Jmeter thread group and iterate over them

I have the following case:
I have four csv files, which contains 2000-3000 thousands each.
All of them have identical rows, for examples:

userGroup1.csv - which contains username, password, secret

userGroup2.csv - which contains username, password, secret

And when I do the perf test I need to send request1 from csv1, request2 from csv2 (using row1 there) and so on.

One possible solution is to have 4 thread groups, each one having their own csv file.
But if I don't want to multiply the thread groups, what are my options?

I was thinking to merge 4 csv files into one, but then if 1 of the csv-files changes, I need to change the merged file as well.

If you have 1 request and want it to read the next line from the next CSV file on each iteration - this is not something you can achieve using JMeter's CSV Data Set Config

Take a look at __CSVRead() function

1. Create filenames.csv file with the following contents:

   userGroup1.csv
   userGroup2.csv
   userGroup3.csv
   userGroup4.csv
   

2. Add CSV Data Set Config to your Thread Group and configure it like:

   

   it means that the the CSV Data Set Config will read the next line on each iteration of each virtual user

3. Whenever you need to read the value from the CSV file use the following construction:

   ${__CSVRead(${filename},0)}
   

   it will read value from the first "column" in the CSV file, if you want to read 2nd column - use ${__CSVRead(${filename},1)}

   when you want to switch to the next line - call ${__CSVRead(${filename},next)}

More information on JMeter Functions concept: Apache JMeter Functions - An Introduction

The easy solution to handle your problem is by creating 4 csv dataset configs and providing different variable names

and in the thread group create different samplers to process the respective csvs - something like

you can see each sampler process the csv according to its variable names