/snippers/
Limiting the natives to TL4 under the guise of "protecting less developed areas from industrialized tyranny" still allows ships and railroads to allow goods to get to market (and off-world) in a reasonable timeframe for the kinds of goods the world produces. The natives can be mollified by a trickle of very expensive luxuries (the expense explained due to the expense of transport). The lack of high-speed information tranmission keeps the many hundreds of cultural enclaves distinct, allowing them to continue producing their distinctive cultural goods (which sell at a premium off-world due to clever marketing, which has created a collector's hobby of identifying which culture a given piece of artwork comes from).
In return for their goods, the TL8 starmen provide things such as TL8 medicine for the low-tech natives but are always quick to explain the evils of "high" technology and how much their own civilizations have been scarred due to the tradeoffs inherent to climbing up the TL scale ("Yeah, technology in our worlds have made all of us independent, so much so that you, grandfather, would not be surrounded by grandchildren and great-grandchildren, you'd have been long since discarded by your own children as 'uncool' and you'd have a long, lonely life in an old people's home, maybe visited on holidays if your children had time with only your advancing decrepitude to look forward to, surrounded by nurses and caretakers who wouldn't be around you if they weren't paid and have no interest in your stories and wish you'd just shut up ... that's TL8 for you.")