I'm going against the current a bit on this one but here's another view on it, take it as you wish
Sub4sub helps you getting views and subscribers which is essential for growing on youtube to gain authority in the YT algorithm. The downside is that they kill your retention and maybe even worse, start a YT view session with your video and stop it from there (not viewing more videos incl other channels). The question is which is worse, and it's a heavy debated point among creators.
Something that everyone agrees on though is that you need to promote your videos. No promotion = no views, they won't come magically by Youtube loving your videos over the other hundreds of hours uploaded every minute.
If you do decide to go down the sub4sub route from a promotion perspective, I would recommend only doing it on channels similar in content and size to yours. This way you increase the chance of becoming friends with other creators in your niche and people actually a little more interested in your videos.
To give you an example how bad promotion can be for your channel I'll use YTtalk as an example
While the place is great and you'll find creators who can help you on your journey to become more successful, it's very bad for your retention from a promotion perspective
Might be my niche I'm in but the average view time on videos watched by people on this forum clicking on the video in my signature is.... 9 seconds
So definitely don't join this forum from a promotional perspective haha (not saying that you want that).