Modulate Bipolar(?) signal
For a signal that rides at VSS/2, goes down to 0V and up to VSS, just use a pair of GPIO pins, each with a $10\mathrm{k\Omega}$ resistor, with the far ends connected together.  When both pins are high you'll get VSS, when both are low you'll get 0V, when one is high and the other low you'll get VSS/2.
Then sequence the pins to get the pattern you want.  If you need something that's faster and has less jitter than you can accomplish by bit-banging the pins, use a pair of timer outputs and sequence them in hardware (which you may not be able to do with the ATMega found in most Arduinos, and which the Arduino hand-holding software may not allow you to do easily -- it's your job to figure that part out).
To get your 2V, 3V, 4V pattern, you'll need up to four resistors, connected together in the middle, and to VSS, GPIO_n, GPIO_{n+1}, and ground on the other ends.  Finding their value takes some algebra, but it's straightforward if you know basic circuits.