Can you please explain how to connect/interface a Raspberry Pi4 and a LimeSdr XTRX.
A PCIe interface is required, hence you cannot.
can you suggest a converter to be used between Raspberry Pi4 and LimeSDR XTRX
You cannot convert USB to PCIe. You need to use a computer with a PCIe interface.
If you would like to use a Raspberry Pi, use Raspberry Pi 5. Then you would need an adapter from its PCIe FPC to mPCIe, which may exist, but I’m not aware of one.
Alternately you could use the LimeNET Micro 2.0 kit:
This takes a Raspberry Pi 4 Compute Module (and when available it should take an RPi CM5 too) plus a LimeSDR XTRX.
The Raspberry Pi people say that are working on the PCIe device. The Orange Pi 5 people claim to already have it.