If you go to
C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto IV\GTAIV\movies
You'll see 5 files, 3 of them can be replaced. CNT, weazel and static.
You just have to convert what ever you want to use to .BIK format using the Rad Tool.
Once converted, rename your new file to 1 of the 3 names and replace.
You could use a program like AntiMacro to bind controller buttons to keyboard keys, and then just use a normal trainer like Simple Native Trainer.
These were included with the "Special Edition" of GTA IV, so you can only find them second hand or buy the whole special edition which includes more things but it's really expensive.
Get CPU-Z https://www.cpuid.com/softwares/cpu-z.html and GPU-Z https://www.techpowerup.com/gpuz/ utilities and run them to find out your exact CPU and GPU and post here. I think your CPU is OK so you may disregard comments about it being old as it is GPU that is much more important for GTAIV.