Ugs What could we do in order to trick the cell phone provider to think the device is old, and it only has 4G on it?
The issue isn't a chess game (or a poker game) between cellular providers and handsets.
Because 5G can support more users with faster connections than 4G, many carriers are adding/improving coverage via 5G, not 4G. As time goes by and a larger fraction of handsets support 5G, it will be rational for carriers to focus an increasing fraction of their equipment investment on 5G.
At some point it will start to make sense for carriers to remove 4G support from areas that have it, because replacing 4G+5G at a specific cell site with 5G-only will allow that cell site to support more users and faster connections. This is different from officially ending support for 4G. Replacing one third of 4G+5G cell sites in an area with 5G-only won't require setting an end date, notifying customers, offering upgrade discounts, etc., because it will still be possible for a 4G-only handset to get some sort of connection most of the time.
The bottom line is that over time a handset configured to 4G-only will plausibly gradually receive worse and worse service, because the extent of carrier support for 4G-only handsets may well drift downward over time. No on-handset trickery can change that: if the service isn't there, it isn't there.