IO with port manipulation while not interfering with i2c I'm using an Attiny84 with the Tiny Core. 
I'm try to use port manipulation to write the lower four bits of port A. I'm using i2c which occupies bits 5 and 7.

How can I write only to the lower 4 bits to avoid interfering with the i2c?
I don't think you need to worry. A simple piece of code:

void setup() 
{
 PORTA |= bit (0);
}

void loop() { }


Generates:

00000044 &lt;setup&gt;:
  44:   d8 9a           sbi 0x1b, 0 ; 27
  46:   08 95           ret


In other words, the compiler generates code to set that bit, ignoring all the other bits.