How can I connect the KT3170 (The DTMF reciever) to USB (4bit data)?
I'd suggest a microprocessor with built-in USB, such as the PIC 18F2455 (or one of its variants). The USB interface to your PC is very simple and ends up looking like an RS-232 serial port to the PC. The driver is probably already in your OS if you're using recent Windows.

The way I interpret the data sheet, I would connect Q1 - Q4 to digital inputs on the microcontroller. Then connect the DSO (pin 15) to a digital input with an interrupt-on-change and process when it goes high. When you get the interrupt, read the Q1 - Q4 data, do whatever pre-processing you may want and send it out the USB port.

I can't definitively tell by the data sheet if the KT3170 output would change if you pressed a second key while the first one is still pressed, but I'd guess that the fist output would stay latched unless you fully released the first key prior to the second press. You might have to just try and see.

If you press one key and then quickly press the next, you should be okay and get the second keypress as the maximum interdigit pause time is 40 ms. As long as you aren't faster than 40 ms between presses, you'll get the second press.

I don't know what your transmit latency will be. You'll have to look for that in the transmitter/receiver pair documentation.