I get anxiety, however, in such moments, consider the facts of the situation and the real probability of something happening.
What was the real probability that a car thief or kidnapper was lurking at your driveway - which appears to be partially hidden anyway - at that EXACT moment anticipating a 10-20 second window when your child would be in the car with the engine running and your DH would be distracted with the bins?
Of course there are the stories of freak type events but factor in that they only make the news because they are rare. Millions of people do exactly as your DH has done with nothing happening. You cannot eliminate every risk in life; the key is to take a rational view on the facts and actual risk (not perceived risk).
And while cars have indeed been stolen with kids in the back, the vast majority of opportunistic car thieves would MUCH prefer an empty car to one with a child in it. So unless your DH's car is something so spectacular that a car thief would target it specifically, if it IS stolen it will likely be an opportunistic bog-standard car thief who would would most probably prefer an empty car they can go racing around in before dumping it. An additional 4 year old is is an unnecessary complication when the thief could just as easily move on and steal an empty car.
Stories from the USA involving cars and children aren't helpful; the vast majority of cars over there are automatic and VERY easy to drive or for a young child to get to where it might roll. Many USA children learn to at least release the brake and put the car into drive very early - some even learning to drive a tiny bit on a private driveway long before they actually properly learn to drive and get their licence. It is a different situation to this.
The reality here is that by the time your child may have thought and had an opportunity to do anything (ie climb out of his seat, make it into the driver's seat and attempt to do anything), it is highly likely your DH would've been back at the car. You don't report your child doing anything like this, so I assume he behaved himself and sat nicely waiting for his dad to return.
Again - I get anxiety and I'm an over thinker... but it does help with things like this to really examine the reality and facts of the situation rather than endless "what ifs" which can spiral and are rarely realistic or probable scenarios. Focus on what is probable rather than what is possible - balance the two out.