Which layer is Forth? xD
If you are asking what level of abstraction is used by Forth, then the answer is: 2nd. It directly exposes stack, memory, and machine code and provides a thin wrapper on top of that. Developers can build higher levels of abstraction then, of course. It's possible to do an object-oriented programming in machine code, because compiler can.
The trickier question is: What level is Lisp? Is it assembler for Lisp machine (because of opcodes like car, cdr, etc.)? Or, maybe, it is a 4GL because of it advanced meta-programming possibilities?
So, it's expected to see an order of magnitude improvement in development speed, on average, when jumping from machine code to asm, or from asm to a 3rd level language. Forth doesn't improve speed of development by an order of magnitude comparing to asm, because of the steep learning curve and low level stack management. I tried to learn Forth multiple times, but still cannot program in Forth freely, which makes it unique among 20+ other programming languages I know.
Chuck Moore created Forth to improves his productivity versus conventional tools and there is some anecdotal evidence that it worked for him and those that worked with him, particularly where direct hardware control on new hardware could be interrogated and validated via the interpreter rather than an Edit/Assemble/Link/Test loop.
[1] - https://www.discoverbooks.com/Forth-The-Fourth-Generation-La...