The maximum number of initiative Passes a character can have is 5, but most character types can only ever achieve 4.
Most
characters may not act in more than 4 Initiative Passes in a Combat Turn (even if they spend Edge
).
...(so a hacker in hot sim with a simsense accelerator and simsense booster cyberware has 5 Initiative Passes). Initiative Passes; this is an exception to the rule that normally limits IPs to 4).
Looking at the SR4A rules, the underlined passages say "max IP's under any circumstance is 5; however, most character's won't see more than 4 and you can't spend Edge to gain a 5th IP". Unwired suggests that one should interpret anything which doesn't specifically allow 5 IPs to be limited to 4 IPs. Therefore, it seems likely that the intention is that an omission translates to a limit of 4 IPs; however, only in one mangled sentence in the back of Unwired does it ever specify that 5, rather than 4 is the case that must be stipulated. Almost every IP boost in SR4A specifies that it cannot grant more IPs than 4.
However, note that Rush only grants +1 IP and you can't buy additional ranks of it for additional IP's. Most powers already state that they do not stack with any other form of IP boost.
For example, the following powers, spells, ability and gear do not stack IP's with IPs from other effects (this is not an exhaustive list):
Improved Reflexes (Adept Power, SR4A pg 196) is incompatible with technological or magical IP boosts, maybe drugs depending on GM?
Wired Reflexes (Cyberware, SR4A, pg 342) is incompatible with all other forms of IP boost (Reaction Enhancers only increase Reaction itself).
Synaptic Booster (Bioware, SR4A, pg 347) is incompatible with all other forms of IP boost.
So, you have to find one of the rare ones (like the spell Increased Reflexes, SR4A, pg 208) that work with other IP boosts, or use drugs, for you to ever get more than +1 IP out of Rush.
