You can use MCUXpresso to develop code for Teensy 4.x, you just have to create a hex file from the .axf file. Just right click on the .axf file and select Binary Utilities and then Create hex. The hex files are compatable with the Teensy 4.x loader. Just be aware the MCUXpresso uses entirely different startup code and linker files, and applications built with it run out of flash by default, not ram like Teensy. Many of the MCUXpresso examples for the MIMXRT1060 EVK board work with Teensy 4.x, but some require tweeking. The MIMXRT1060 EVK board uses the IMXRT1060 MCU same as Teensy 4.x, but with a different package, however as far as I can tell the pinouts are the same for both parts. The MIMXRT1060 EVK is a MUCH different board than Teensy 4.x, so for many of the MCUXpresso demos the hardware is different, so you have to adjust for Teensy using the MCUXpresso Config tools. I use the MIMXRT1060 EVK board and the MCUXpreso with its nice debugger then port the code to Teensy. It's a nice IDE but as mentioned getting help from NXP community forum is sometimes difficult and some of the NXP documentation is good but some of it isn't and a lot of it is hard to find. The PJRC forum is many times better!
John