Just a small word of warning.
Flashing modems on the S4 isn't quite as innocent compared with the good old days of the S2 where we would hurl our cac-covered flashing boots at it and still come up smelling of roses.
As yet, modems cannot be flashed via CWM, only Odin. To me, this poses the question "why not?", so you should be cautious from the offset. Even then, odin only flashes a modem properly if the device is put into download mode through a power-down and not through the power menu. Again, alarm bells ring at me.
Personally speaking, I would not recommend flashing modems on the S4. On the S4, modems seems to be more tightly linked to the firmware version for which it is intended. Flashing alternatives is not without risk and can lead to serious problems like wifi and sound disappearing. Getting these back to a working state has been a very long winded and laborious task for quite a few unfortunate people, with cwm restores and even reflashes of stock firmware not being sufficient to return to the status quo.
Of course, if you have nads the size boulders, akin to ironass' then this will not represent a problem to you, but to the mere mortals amongst us, its not something that is to be taken on lightly.
I really don't think its worth the risk considering the minimal (if any) "benefits".