I found red fluid in the rear master cylinder resevoir, I assume it's power steering fluid, do I have to replace all the seals like with dot 3 or is dot 4 different since it has a different formulation?

If you are in doubt about what is in your master cylinder, drain it and then refill with dot 4.
You may want to drain and replace it then ride for a couple of days then repeat the operation to be certain that all the old fluid (or power steering fluid) is gone.
It's only time consuming as the brake fluid is quite inexpensive.

Thanks for the reply, I'll start on that tomorrow.

Power steering fluid is oil and if you had it in your bike all seals would be stuffed with no brakes at all as they all would have swelled and failed. DOT brake fluid is not an oil so does not affect rubber so unless you put PS fluid in or someone else did in the last few days it is just red brake fluid. ALWAYS and yes I am shouting change all fluids and filters when you buy a new to you bike or car that has been owned before.
Regards,
K-BIKE

I have never seen red brake fluid. I sure it was ps fluid because the rubber under the cap was swollen. The reason I noticed the color was in fact my intention to replace all the fluids in the bike.

Then if you are in any doubt and suspect it is PS fluid do not pump it through the brake system. Dip your finger in it and run it under the tap if it is oil it will not rinse off, if it is brake fluid it will. If it is oil then I would advise replacing all rubber parts in the system, including sadly the hoses because they will likely be affected if the fluid has been bled through the system. So do the dip and rinse test ASAP.
Regards,
K-BIKE

I know that on oil intrusion in automotive brake systems all the rubber components have to be replaced( dot 3 systems). I was unsure about a system that had dot 4. The rear already has had ps pumped thru it so I am replacing all the rubber pieces.