How much print resolution will you gain using the new Sony A7RIV? This chart will give you an idea. Print height and width are listed across the top, and the columns show the ppi file possible from a specific megapixel sensor.
A quick look shows us The 61MP sensor of the Sony A7RIV lets you make a 20×30 with 316.8 ppi of data, a 51 ppi increase over a 42MP sensor at 265.8 ppi, or about a 19% increase. Both will make exceptional 20×30 prints, but the 61MP sensor will resolve more fine detail.