@Arcangelus
True, Poland wasn't known for its high population density or urbanization rate during the middle ages. But it was known for its economy based on agricultural surplus. Also it compels players to use Polands UP (yay! got the abbreviation right this time). Spread the religion to supply the potential additional population with happiness.
Nevertheless, I'm curious if you have an alternative in mind.
Let me state the facts:
- Poland (and by extension, The Poland-Lithuanian commonwealth) were known for selling agricultural products on mass. (mainly grain, if I remember correctly)
- Poland wasn't high on the population density department.
- As such, producing more food didn't help Poland population wise, but economically.
- Eventually, the prices fell and Poland's economy followed suit.
- The attempt of Poland's nobles to increase this kind of economic model led to further damages.
- The model existed in one way or another until its dissolution by the communist.
Now, on civ4:
- More food means more population.
- More population equals more tiles that can be worked.
- More tiles being worked leads to a higher production, commerce and food.
- Besides food, the main limiting factor on grow is happiness. Having a high population is not that helpful if most of them refuse to work.
- The Polish UP allows them to create extra

buildings, allowing usage of citizens that for anyone else would be

for most of the game.
- The final factor limiting grow is healthiness. You can offset it with a higher production of food, which is what you suggest.
- The combined factors result on high density, highly productive cities that are objectively better than what anyone else can get (except maybe Kiev with religious tolerance).
The history would suggest a gold(money) bonus, not a

one. But again, I'm not sure if Poland needs a buff on that area.
Do you imply that the original idea seems insensible? (And per extent this idea also seems insensible?)
If so, I'd like to learn why.
I don't imply. I'm saying: under the current circumstances, Poland can get a high population already (if they bother to develop their land, that is). Last time I conquered them as Germany, I managed to get a considerable population from that area (and I had to deal with low stability and the mongols in the process). Giving them a bonus on food would increase their population further, and their UP allows them to use more of that population (the religious buildings that give

translate into extra useful citizens that most civs can't afford until much later). More useful citizens means that Poland is stronger in all metrics, which is not quite what happened.
I said that the second idea was more sensible because it doesn't escalate. +4

means 2 extra citizens. Strong, but there it ends. You can easily get more food from your original idea, which translates into more citizens that can benefit from the bonus.
How much do the alternative strategies differ from intended (through UP and UHV) gameplay?
Personally, I deem this point as irrelevant. If I want a domination victory (for instance), any power that helps me with stability will be welcomed. After some practice, is not that hard to collapse most AI nations nor make them receptive to capitulation (in essence, kill most of their units and take a city or 2. A few blockades also help).
As Poland, you biggest problem will probably be Germany; however, there is little resistance to eastern expansion (or to Lithuania). I'm pretty sure to have read a post about how to archive this type of victory with Poland somewhere (and Germany indeed attacked in there, so he had to strike first).
You are up against some very solid logic here. As long as missionaries cost hammers you should just consider yourself convinced. Move on to an argument you can win, like the pros outweigh the cons of this bonus.
This is an excellent argument for stating why you think the Polish UP is good enough as it is. The problem with it is that besides more options to do good things you also have more options to do bad things. Now, more options to do bad things, is that a bonus? (I daresay not) So at best this application of the Polish UP is a mixed blessing.
Actually, an ability that allows me to make bad decisions is great. It means that the ability grants me a bigger margin for error, and thus i have less need to play "perfectly".
Besides, if your biggest issue is the "dead time" while creating missionaries there is a better bonus for that situation. Simply add a bonus for missionary creation. Let's say 50% cheaper missionaries.
(although I'd be surprised if you weren't prevented from doing at least some of those things by other things than religious instability. If someone who is dying of cancer gets hit by a truck, did the cancer kill him or the truck? - I daresay the truck, implying that the bonus is not as enabling as you make it seem to be.)
It may very well be the cancer.
Was the person in question in severe depression? If so, did the depression make him/her suicidal enough to purposefully go into harms way?
Was s/he paying little attention to the traffic because of the cancer (directly or indirectly) and tried to cross when s/he shouldn't?
Was the driver of the truck related (by blood or otherwise) with the cancer patient, and did this fact distract him long enough to hit the person in question?
Was the cancer impairing his sensorial or intellectual capacity?
Did the treatment weaken him/her enough that wasn't able to react in time?
Was the cancer causing an aneurism when the patient was into the truck's way?
Did the cancer cause a multi-systemic failure in the street, being hit by the truck after being dead?
Those are just the few that I can convey properly in english. I'm sure there are many more: as long the remotion of the cancer from the equation result in him/her not being hit by the truck and/or being dead, you can consider it as the cause or catalyst of the situation. Remember that the Occam's razor doesn't say "the simper answer is always correct" but "the simpler hypothesis that fully answers a given question is the more likely to be correct". But this is nitpicking, so carry on.
My personal take on this issue is: If improving the UP of Poland worsens the UP of Kiev, you might want to reconsider the UP of Kiev.
But you can get quite far with line of thinking. And by that I mean that, buffing a UP will result into easier UHV (assuming no changes). As that is not desirable in most cases, the difficulty of the UHV will be increased. This can lead to a relative weaker position to achieve the UHV to the prior situation even with a stronger UP.
Now, if the relative power of a civ directly affect others, the impact of this line of thinking has more a profound impact. Kiev in particular almost doesn't interact with anyone else, so it's not relevant for this. However, Poland interacts with at least 3 other civs (Germany, Lithuania, Prussia, maybe Hungary and Novgorod) and it interferes with at least one of their UHV. Because a much stronger Poland means that their survival rate is decreased and/or the UHV gets more difficult to archive, you may want to increase their relative power as well. It's difficult to determine when enough is enough with this logic, and the work involved may not be worth it.