Those who participated in the Fourth Crusade did not originally know that it was an expedition against Christian lands. As you note, it only became so due to the deal struck with the Venetians, which no ordinary crusader anticipated. Originally it was intended to attack Egypt, with the crusade then continuing to recapture Jerusalem, and this is what one may expect somebody going on the crusade to have expected. Look at preachers recruiting for the crusade. Take, for example, Fulk of Neuilly. Mayer (The Crusades) has claimed his addresses were focused almost exclusively on issues of morality and sin. This is not to suggest that crusaders signed up purely with these ideas in mind; the point is there was certainly no mention of attacking fellow Christians. Even when the crusade was diverted to attack Zara, it was expected that the crusade would continue to the Levant/Egypt. To the common crusader, there is therefore no reason for them to expect this crusade to be purely an "expedition against Christian lands".

And even when the Pope excommunicated the crusaders and demanded they return to the original goal of retaking Jerusalem, the leaders of the crusade hid this from their fellow crusaders (Runciman, The Kingdom of Acre and the Later Crusades, p.98). It would therefore be wrong to assume that the vast majority of those who took part in the Fourth Crusade were originally motivated differently due to knowledge of the different nature of the crusade. They simply did not, and could not, know the path it eventually took. Many crusaders certainly deserted during the Fourth Crusade, but I can't prove that this was due to disillusionment with the direction the crusade was taking. After the events of 1204 some crusaders even continued on to the Levant. Certainly it was not many; 5-10% seems a reasonable estimate (this is simply to give an indication of the low numbers; Jonathan Phillips claims one in ten).

In terms of having less morals? It's hard to take the other crusades as examples of moral impeccability. Take the Siege of Maarrat in the First Crusade, well known as an example of crusader barbarity, with documented evidence of cannibalism (Runciman, A History of the Crusades, Vol. 1, p.260-1). The 1204 sack of Constantinople was undoubtedly barbarous, but should not be seen as unique - the 1099 massacre of the inhabitants of Jerusalem is an apt comparison. It's hard to believe accounts of crusaders "wading in blood" (Raymond of Aguilers), but nonetheless the morals of crusaders in these crusades were not diametrically different.

The motivations for going on crusades have been widely debated by various historians and there is still no complete consensus. It is very difficult to quantify motivations. Nonetheless, "glory and fame" certainly contributed to the motivations for many crusaders in all crusades, and should not be seen as unique to the fourth. Likewise, as shown, glory and fame were not the only motivations in the Fourth Crusade. Thus it would be hard to claim that the crusaders were motivated differently because the crusader turned into an expedition against Christian lands.

If you're interested in further reading, I would strongly suggest Thomas Asbridge: The Crusades for a semi-casual reader. Very well-written and accessible. For more focus on the Fourth Crusade, Jonathan Phillips: The Fourth Crusade and the Sack of Constantinople may be the place to start.

Sources:

Phillips, The Fourth Crusade and the Sack of Constantinople

Runciman, A History of the Crusades

Runciman, The Kingdom of Acre and the Later Crusades

Mayer, The Crusades

Edits: Spelling and clarity, reading