Welcome, Monika! You'll likely get several responses here, most of which
will be more helpful than this one, but as I'm the early bird here
today, I can give you a couple of things to get started until the folks
who really know what they're talking about show up.

> Unfortunately car roof obscures view of the sky and over 95% 
> of time the receiver cannot get a lock on enough satellites. 
> (Placing it on a dashboard did not help). Do you know of a 
> way to get around this problem?

I don't know the Magellan 4000, but the resource I checked states that
it has a receptacle for connecting an external antenna. These connectors
are usually one of a few standard types, so it should be possible still
to get an antenna that will work with your 4000. They can be obtained
with magnetic bases, so you can put the antenna on the roof of your car,
and run the cable through the window to your GPSr. This solution, of
course, costs money.

> Do you know where I 
> could access good quality on-line maps with latitude, 
> longitude and altitude?

Your best bet is probably TopoZone (www.topozone.com). Just be aware
that those maps use the NAD-27 datum, and your GPSr is probably set up
to default to the WGS-84 datum. Most GPSr units offer the ability to
convert between datums, and the difference between the two can be
significant.

> If not, then do you know of a 
> bookstore or another place in Phoenix area, which would carry 
> good quality cartographic materials (which are not terribly 
> expensive)?

Don't know about expensive, but the most complete resource IMO is Wide
World of Maps (http://www.maps4u.com/). They have two stores in the
Valley, one on Indian School near I-17 and one near Fiesta Mall in Mesa.

Steve
Team Tierra Buena