The Operation Manual for Dorico 4 certainly does describe the new Insert mode functionality. You can find it if you start reading here:
This page also includes a short embedded video that describes the changes to Insert mode in Dorico 4 comprehensively.
If you have a reproducible case of a problem – whether with Insert mode or indeed anything else in the software – then you should provide the project (ideally cut down to the smallest section that reproduces the problem) together with the steps required to show the problem. You can either post it here, or you can send it to me directly, either via a private message on the forum, or to my email (d dot spreadbury at steinberg dot de). This is all described in one of the sticky threads here on the forum, which I will link here for your convenience: