Phone manufacturers have mostly moved away from the 3-4 inch phones. So why do you expect Apple to continue to stick with the tiny 4 inch phone? I understand that some Apple users prefer the super small form factor of the iphone SE from 2016 and iPhone 5S from 2013. There are probably some andrioid users that want a small 3-4 inch phone but they have to adapt. If they don't want to adapt to bigger phones they have to keep a much older android phone or go to an older iPhone if they want a 3-4 inch phone.
At least Apple updated the iPhone SE to give us the SE 2020 edition which includes alot of cool new features( check my other posts in this thread if you want that info). Sure the form factor of the new SE is slightly bigger going from 4 inches to 4.7 inches. However, it is still a somewhat small smartphone especially by today's smartphone screen size standards.
Take a look at all the phones currently being sold on T-Mobile's website. They have over 35 different phones for sale from various manufactures like Motorola,Google,Apple,Samsung, Alcatel, LG, ONE PLUS etc
All the smartphones they have for sale currently on T-Mobile's website are in the 5-7 inch screen size range. With some exceptions: They currently still sell the iPhone 7, iPhone 8, and the iPhone SE 2020 edition all of which are 4.7 inches which are the smallest phones they offer for sale in the smartphone category. The only thing phone they sell thats smaller that the 4.7 inch iPhone 7,8, and the new SE is the Alcatel GO Flip 3 which is a flip phone. Click on the link and take a look for yourself.
https://www.t-mobile.com/cell-phones. Or go to T-Mobiles website and click on phones and devices and then click cell phones from the drop down menu.