I hear you about the bike trailer and trail-a-bike taking up space, but I don't see a way around your cable issue, so I'd suggest reconsidering the trail-a-bike.
My 4 y/o rides a trail-a-bike and really loves riding a "grown-up" bike. It's super fast to get him on and off (he can get his helmet on and climb on the bike by himself), and the attachment is also pretty easy. If you have a space to hang it on a wall, a trail-bike might take up less space than a seat (unless you're going to keep the seat mounted on the bike all the time). This is also a potential longer-term option, as he'll be able to ride the trail-a-bike for years on trips that are a little to far on his own.
On safely, my version has a rear flag, and the bike doesn't trail quite as far back as our trailer, so it's pretty visible and I've never had an issue. (My older child also rode the trail-a-bike starting at 4.)
I know that 4 is a little young for most trail-a-bike recommendations. My son is shorter than average for his age, but can still reach the pedals. He does have better balance than average (rides without training wheels on his own bike), but I don't think the kid's balance comes into play as much on a trail-a-bike.