My msitake,I'm stuck in last edition as always. The -1 to hit in combat does help quite a bit, but when you consider that the main threat to him is cannonballs, other war machines and shooting in general, the majority ofwhich he would only be getting a 5++ against, I might consider the 4++. Although it does help out a lot in combat, the altar is fairly durable without, so is not under any immediate threat while the lector has a good armour and in the situation I'm describing, a 4++, he should be able to survive pretty well on his own. as well as this, it gives him a couple spare points for a magic weapon which he desperately needs as I stated in my long post (sorry for ridiculously long post btw) for etheral defences, especially for the altar since its a nice juicy target for hexwraiths, wraiths and even spirit hosts to hold up and maybe even cause damage.

Either way, I'd say try them both out, see which one works better or which one you like and use that one. Personally, I'm for 4++, but no-one's going to judge you if oyu take white cloak

Sorry for long post again, I feel the need to elaborate on points way too much