Since the ukulele has only 4 strings, which is less than the guitar, does that mean that some pop songs are not able to be played on the ukulele?
Any song can be adapted to be played on any musical instrument.*
That having been said, some musical instruments have more limited abilities than others, and require more adaptations from the original. And it's a matter of opinion how far you can go adapting your song to a different instrument until it is no longer the same song.
As you mentioned, a ukulele can only play 4 tones at a time, compared to 6 on a guitar. In addition, a ukulele's strings are tuned to a higher pitch than a guitar's, which means that guitars can play lower notes than are possible on a ukulele. So if your question is whether there are pop songs with guitar parts that cannot be duplicated exactly on a ukulele, then yes, most guitar parts cannot be played exactly the same on a ukulele.
However, there are corresponding ukulele chords for most guitar chords. These chords do sound different on a ukulele than they do on a guitar, but can be played in a song and sound just fine.
* I'm sure someone could play Beethoven's 5th on a triangle (at least the rhythm), and a few people listening might even be able to name that tune.
You can give a reasonably convincing version of just about any style of music on uke. Here's just two of my friends proving the point - Andy Eastwood with his rendition of 'William Tell' overture, and Mandi Harkett with her Opera-lele
Pop songs? No problem!
The big problem with converting guitar music to ukulele is the voicings that guitar uses. Any guitar chord van be voiced on ukulele. Some, however, are more difficult than others. Even extended chords like C13 can be accomplished on ukulele rather convincingly, but often the problem is how to connect these chords smoothly, with voice-leading and some chords being hard to invert on ukulele.
Ultimately, it comes down to how accurate you want to be with any arrangement of a song. It's unlikely that you'll be able to play the exact same chords as the original guitar, but that shouldn't mean the song is impossible to play. You'll never be able to perfectly recreate a pop song on any instrument. But that doesn't mean it's impossible to arrange it, you just have to be creative in how you make the sounds you hear using the restricted string set of an ukulele.