I wouldn't assume that the SE 4 will be as small as the SE 3. The SE line is built on recycled older generation iPhone tooling, and iPhones in general have trended larger, so as the SE line catches up to newer tooling it too will get larger. It already happened once between the SE 1 (iPhone 5S derivative) and SE 2/3 (iPhone 8 derivative).

If the SE 4 has an OLED screen then it might be derived from the iPhone X, since that was the first OLED iPhone. That would be a size increase from the SE 3.

https://www.phonearena.com/phones/size/Apple-iPhone-SE,Apple...