Theory Question

I’m working in an AP Music Theory book and have a question that someone else with a better knowledge of music theory will probably be able to answer. In my book when talking about many things they use the four voice types as examples, but I write for instruments and don’t know which instrument would go with which voice type. Like if I’m writing for flute, clarinet, trumpet, and alto saxophone (this is just a random example), how would I know which instrument would go with which voice type and get the melody? 
 

I thought that maybe it was the instrument’s range but I’m not sure.

Thanks

having a good knowledge of the different instruments' strongest ranges is key to assigning voice types to instruments. the instruments' ranges definitely play a part in this, but remember that ranges between instruments will often overlap with one another. I'll use your instruments as examples:

the flute excels in ranges that extend above the treble clef staff, and will have no problem piercing through the rest of your instruments at its highest registers. for this reason, it'd be good to assign it a higher voice

the clarinet has a beautiful, soaring quality in its higher registers (not too high!), and a deep, rich, tone in its lowest registers. it sounds a little bland in the middle of the staff (sorry) so it'd be nice to put it in its stronger registers for best effect.

the trumpet has a great lyrical quality in the mid-high portions of the treble staff, and it is quite taxing to play above the staff for extended periods of time. at the same time, it struggles with lower notes especially in softer dynamic levels.

finally, the alto sax is quite homogeneous in sound throughout its register, and a good player can do well in all its registers.

basically, do some research on what range works best for your instruments, and what tone quality can be achieved in these registers, and use that knowledge to pick a voice for your instruments.

What your asking is more of an orchestration question than theory.  As the above commenter states, knowing the ranges the subtleties in timbre for the various instruments you are using is vital.  It would be worth referring to a good orchestration text.  I personally use the Walter Piston text which is pretty readable and has great examples.

Yes, I aggre Walter Piston is a great book for orchestration. I have it and use it quite often. It includes a part about orchestral uses for each instrument. 

@Rileyt: your question is not so easy to answer. A first approach is of course considering the register. But also the sound qualities are important, or the blend with other instruments if you use them in an orchestral context. Also woodwings have a different sound quality depending on the resgister: a clarinet can sound very pearcing in the high register and very warm in the low register, while strings are more homogenious in all registers, etc...

If you are thinking in four voice arrangement in the singing register like an SATB choral the combination Oboe - Clarinet Bb (or E. Horn) - French Horn - Bassoon works pretty well. I wrote some choral-like pieces with this arrangement, but also many other combinations are possible. 

You can have a look at the link of Vienna Instruments, they have a nice section on instrumentology with a lot of information: 

https://www.vsl.co.at/en/Academy/Instrumentology

Me personally, I think a Flute, Trumpet, Alto Sax and Clarinet group is a poor choice of instruments if it just going to be a chorale.

Again, me personally, I think a more appropriate choice is Flute, Oboe, Clarinet and Bassoon. or Trumpet, F Horn, Trombone(or Euphonium) and Tuba. Or what @Guillem82 suggested: Oboe, clarinet, french horn and bassoon.

But like what @bkho said, that is a orchestration question, not a theory question. 

just my 2 cents

As others said, orchestration and not theory, and as the guy above said, it's not a good choice anyway.

The important thing in harmony is homogeneity; so you want to use instruments that have the same or very-similar timbre.

Woodwinds are sorta the worst at it because they don't just sound very different from each other to begin with, but different from two of the same instrument depending on range. 

Flute, trumpet, alto sax and clarinet can sound wonderful together. It's about the effect you want.

Even if you want a homogenous texture there are many tricks like bucket mute on trumpet, combining the register of the alto sax with clarinet mid/low register, high register clarinet with flute, trumpet with saxophone and clarinet middle registers. Although, for your first arrangement / piece I would not choose this, because it is complex.

Look at instruments that are often used together: string section, brass section, groups of the same family like clarinet quartet.