Yes, Grien-&-bare-it. (Or is it Grinder?) Welcome to TalkBass ... ...
the main thing is to compare similar types of ratings. That is RMS & Continuous watts versus Program or Peak watts, which use different scales. 
... ... As JakobT has already pointed out, the Rumble 210 is rated 700 watts Program (very short time), but
also 350 watts Continuous (long time). The 250 watts minimum marked on your R500c is Continuous. 
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Indeed, Grinder, you seem to be the targeted market! Even when corrected (twice by JakobT & I) that
the actual, usable rating for the 210 cab is 350 Continuous watts, you still cling to the marketing (very short time) 700 Program watts! Likewise
the 410 is 500 watts Continuous and 1,000 Program. Those Program numbers are only good to know because the
more usable Continuous numbers are exactly half.
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Oh, come on, Grinder! You're still mixing Program watts with the more usable Continuous watts! It's like mixing miles-per-hour with kilometers-per-hour, the math doesn't work!
I'm gonna correct your message here for you, to get the ratings straight and make the math work.
... ... I have an 800 hd head coming in, I also have a V3 4x10 cab coming in. I already have a V3 2x10 cab. (8ohm, [350W Continuous], 700W Program) Assuming the new cab is 8 ohms, [500W Continuous] (1000W Program) the total impedance will be 4 ohms.
Back of the head says "Rating: 400W @ 8 ohms/ 800W @ 4 or 2 ohm minimum" ... ...
The ratings marked on the back of your Rumble equipment is in Continuous watts, which is half the Program watts that the marketing headlines give you. Do the research to get and use the Continuous watts in pairing the equipment correctly.
The 210 can easily be safely run alone with the 800 HD by keeping the volume down. The head, only when fully cranked up, can output up to 400W Continous into that single 8 ohm cab. But, although this is technically over the formal 350W Continuous rating of the 210 cab, it is the exact same speakers that Fender builds into the Rumble Stage 800 combo, which can also output up to 400W Continuous into its speakers. Therefore, apparently Fender is confident, from it's testing and field experience, that the 210 cab can safely handle up to 400W Continuous! It is anyone's guess why they haven't formally changed that rating from 350W Continuous to 400W Continuous.
The Rumble V3 410 is, indeed, 8 ohms & 500W Continuous. So, it will have no trouble taking the 400W Continuous that the 800 HD can throw at it. So, to get the full 800W Continuous out of the 800 HD, the 410 and 210 would be fine, since each can handle up to 400W Continuous. Since they are both 8 ohms, they each get half the 4 ohm load.
... ... On paper according to fender manual, the 2x10 can only be used by itself. ... ...
Whoa, right there! I don’t find anything like that in
the Rumble 210 cabinet manual. There is no problem using 2 of them with a 4 ohm amp, or 4 of them with a 2 ohm amp (like your 800 HD).
Please quote what you're reading that seems to say this.
... ... the 2x10 ... It's rated at 8 ohms & 350W Continuous and 700 watts Program but the Amp says 800W Continuous minimum at 4 ohms, which is what I'll have if I connect the 4x10 8 ohm cab in addition to the 2x10. ... ...
Yes, connecting both 8 ohm cabs to the 800 HD gives it a 4 ohm load and can draw up to 800W Continuous from the amp. Fortunately, each 8 ohm cab can only receive half of however much the amp is putting out. And so, in this case with a 210 and a 410, there should be no problems, as explained above.
... ... Is it strongly recommended to not use the 8 ohm 350 watt Continuous, 700 watt Program, cab when minimum power rating is 800W Continuous at 4 ohms, or is it done frequently? ... ...
No problem, in this case, as explained above. The most the 210 cab can receive, whether alone as an 8 ohm load, or with the 410 cab in a 4 ohm load, is 400W Continuous. And the manufacturer is apparently comfortable with that, as explained above.
Of course, you can always turn the volume down from the maximum, and probably always will, which will send less than the maximum power to the speakers! Some people say that a half volume setting sends 75-85% power to the speakers, so it is not a linear relationship. But, experienced musicians can often safely run underrated cabs with high powered amps by keeping the volume down. (Not that this is necessary in your case.)
... ... Thanks and Happy New Year to You !!!
Joe
Same to you and yours, Joe.
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And a double tie would, pro-ratably, award all those entries with the tying categories in 1st & 2nd place, in either order. Or do I hear any objections to this? 
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