I'm curious how linearly GTAIV can be played... I know that it is primarily intended to be an open world sandbox game but I'm curious if one were to follow the story carefully, can it be played in a linear fashion?  I didn't like the previous installments and I haven't enjoyed other open-world games like crackdown or godfather so I'm not going to get it until I know that it is something I'll play and enjoy.  Mass effect is a good example of a game that can be played a bit more "open" but can also be very linear if they user sticks to the story... 

You don't have to vary off and do other things, though you will keep on getting phone calls from those people who you ignore. If you want, you can just jump straight from story mission to story mission, and you can get around nearly instantly by hailing a cab, and that can take you to the important destinations

Well if this game has mission indicators, like the last game, just find the mission starts on a map.

If something else, then just grab a strategy guide, and just look at the parts that tell you where the story missions are.

You can use cabs that way in this game? Cool. I'm more of a story player myself, and that will make it a lot easier. 

Basically, a story mission appears as a letter on the map, and for everything else you need to phone up a contact to organise it.