long story short; the price of the upgrade is dependant of the card you register it to. In essence, that means since you have a 4 dsp card you are only obliged to the cheapest option. However, that is far from a showstoppr. If you decide to buy another card later on, you can just hook it up to your P1, and the additional DSPs and I/Os will be available to you with no further purchases being nececary.
However, what i ask you to consider is that
a) If you are happy with the ASIO latency of the P1, then you cant go wrong in upgrading that card to 5.1.
b) If you plan to add more cards, consider buying a second hand second generation card with 5.1 instead, as that elevates the price/performance ratio.
c) the 3/4 dsp description is not a limit as such, it only relates to how many dsp there are on the card you register it to. as an example if you buy the 5.1 upgrade for your 4 dsp card and add a 15 dsp card afterwards, your system will be a 19 dsp* system running 5.1 with more I/Os.
d) your 4 dsp card will be a bottleneck in a multi card setup, not only for latency but it has less PCI throughput. This might not bother you unless you use a lot of instances of reverb or other delay-hungry stuff.
* 19 or 18, even that has variations but its irrelevant to this issue

*
EDIT: GaryB clarified while i was typing, that basically means that if you plan to keep your 4 dsp card as part of your system, go buy that upgrade.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort.