And, FWIW, that 4k to 5k primary is more typical in post 1962 or so guitar amps than any other tube gear. And early guitar amps often used values more typical of other gear. 6600 is likely the most common primary impedance for a pair of 6L6's. But Fender and others found that they could get more power (at elevated harmonic distortion content) by using the lower 4k ish primary impedance. It's a slightly different feel and sound between the two. But just think tweed era vs. BF era Fender and you'll get SOME of the idea. Of course there were many differences between the preamps in either era too.
6V6's like pretty much what 6L6's do. Except that 6L6's handle lower primary impedances with more aplomb. When guitar amp companies started running 6L6's at 4k primaries they seemed to keep higher numbers for the 6V6's. But for most amps up to, again 1962 or so, 6L6's and 6V6's both used about 6600 primary for a pair. So, if the above "example" tranny were determined to be good for a pair of 6L6's at 8 ohms for a 6600 primary, there is no reason the same tranny couldn't be used for 4 6V6's into a 4 ohm load with a 3300 primary. Get it??? It's all about the turns ratio and the capacity of the transformer.