I have have Galaxy S4 (GT-I9505) which up to this point has run perfectly on Vodafone 4G and still does.
The issue arises when I put in my Telecom Prepaid 4G sim the LTE option under network selection disappears, I put my Vodafone sim back in, it shows and I can connect to 4G again.

I was speaking to somebody who also had this issue, Vodafone version of the S4 which works perfectly on Vodafone 4G but it appears the use of the Telecom sim disabled the LTE option from the network menu.

Naturally if I bring up the *#*#4636#*#* it will show the LTE option like it does on most phones, but if I select it here I can successfully get on Telecoms 4G.