DS aged 4 can communicate some, but much of his talk is absolutely incomprehendable, why do they do this?

[yukoncher]

Me and DH used to joke 'oh my gosh, we've figured it out, our child is foreign!' it would be the perfect explanation.
Much of his talk is nothing like anything. Maybe you'd expect very poorly pronounced english, but this is not even english.
I think he's created his own language.

He needs to also stuff a load of sounds together that are similar to words, and pause quite seriously for me to answer him, so he takes his talk seriously, it's not necessarily play.
It's as if he's enjoying immitating language in general, not necessarily English, though.

Anyone else experience this with their kids?

He tends to also...

It's so interesting, yes it must be jargon.

It's about the same lengh as a sentence, has a normal about of syllables per 'word' said in the right tones.
No idea what any of it means. And I mean absolutely no idea, and I listen carefully as does DH. It's nothing like english.

But yeah, DS is doing well too beside this. So I'm really proud atm.

Ah, hmm.
The few lines of English he knows have been very very rehearsed

I was going to suggest dyspraxia as well...

The 'jargon' thing makes sense; DS(also4, S&L delays) fills in his speech with gibberish/jargon/martian. It's of a similar length and has the same rhythm/phonemes as the missing words.

Your son may also have issues with auditory processing (Sensory Processing Disorder). He's hearing words correctly, but isn't able to decode the information. this also extends into auditory feedback from his own speech.

Then there are the verbal stims and tics...

My son (4) does this too. I think he wants to engage and communicate but doesn't know what to say. He also makes up words that are approximations of real words. I take it to be a good sign that he's trying to get to get to grips with language.

DD (4.5, SLI with severe word finding/memory problems) does this too although now less so as gradually some words are just about recognisable to me and people who know her very well.