I have at the moment a circuit using an Atmega328P that sleeps using the timer and consumes ~4.5uA in sleep mode. Is there any way to get that lower? Is there other Arduino based chips that can sleep with a wake up timer better than an Atmega328P?
Its in 3.3v 8Mhz mode.
Thanks!
FYI I read everything from Gammon already 
Why not read some datasheets of the other atmel chips?
Because sometimes there is some else on the forums who has already done some amount of grunt work. Not all Atmel chips are Arduino compatible as well.
You should be able to get it a little lower than that. What do you have for the circuit?
Have you read Nick Gammon's power usage guide?
I have, the extra 1uA is from a sleeping xBee transmitter.
I see these bluetooth modules that can sleep and function under 1uA and I hope to get something closer 
Attiny 24-44-84, 0.13uA in sleep mode and WDT disabled and VCC = 3V.
Attiny25, similar.
Attiny2313, similar.
Atmega1284P, similar, 0.15uA. WDT enabled, 5uA.
In sleep mode with WDT disabled, nothing is switching internally, so no power is consumed.
So basically the numbers I am getting with a WDT enabled is about right across the board ?
Is there a way to use the 32Khz Osc instead in sleep mode?
If things are switching, you're not going to get lower.
Power in a digital CMOS chip is proportional to clock frequency, how many gates are clocking(*), and supply
voltage squared. (plus some losses from the crystal oscillator and analog circuitry in practice)
Thus the 3.3V 8MHz ATmega running normally consumes less than a quarter of the power as at
16MHz and 5V...
(*) actually the total capacitance of all signals / gates that are clocking, thus f C V^2
By the way trying to get the current consumption below a few uA may be fairly pointless - the leakage
current in your battery will start to dominate at some point, so you may be making no further
improvements to battery life. 5 years at 5uA is 220mAh of charge (pretty small battery), yet some
battery technologies self-discharge on faster timescales than this.