Newbie needs advice

I'm not sure there's a best-for-beginner answer here. Maybe your instructor could clarify his suggestion.
I expect any choice works here; it will get you thinking in terms of chords as opposed to single notes.
It will also give you some insight to what other people are doing if you get to play along with a group.

Soprano is certainly the "classic" ukulele, but they all have their individual merits.

FYI, there's plenty of good posts accessible via search, but you may need to search using google using eg:
google.com: "site:ukuleleunderground.com/forum baritone"
to get access to posts predating 2010, due to a site update.

Soprano, concert and tenor are generally tuned the same (gCEA) and sound the same notes, but since they have different size bodies, they each have their own "sound". The longer the scale, the more tension there is on the strings. The baritone is tuned like the four upper strings of a guitar (DGBE) and is less ukey sounding.

Since you're already playing bass, tension shouldn't be an issue. Some guitarists favor the tenor (for its ukey sound over bari) because the fingerboard is roomier than that of a soprano.

ameliasshow said:

I have been learning to play the electric bass guitar and my instructor wants me to begin learning the ukulele also. I have done some research and learned that there are soprano, concert, tenor and baritone ukuleles. What are the differences and which one is the best for a beginner?

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Wow! Big font!

The first thing that comes into my head is: do you want to learn the 'ukulele? Because it will never happen if you don't have any desire to play it or any other instrument. If you answer yes, then know that there is no right answer for size. But if you are coming from bass I would think that the biggest frets and scale would be best - meaning tenor or baritone. A baritone is tuned just like a guitar's bottom 4 strings the others are tuned 5 half-steps higher: GCEA. I play a tenor, but just pick what you want. If you invision yourself with a small 'ukulele, go for the soprano. And vice-versa.

The downside to baritone that I perceive though has to do with if you like songbooks that are instrument-specific.
Most contemporary ukulele songbooks are written for the GCEA tuning. Discuss with your instructor what she or he envisions for you as far as how and what you'll be playing.