People will spout the trite excuse that "4 men is the recommended party for tabletop sessions".
But that's a pointless "apples and oranges" comparison. P&P with 4 people to consume in short bursts and a single player campaign sprawling across 80-100 hours are two very different things and beg for a very different degree of variety.
Meaning that, yes, a party limited to four is abysmal for variety.
That said, we already had a good dozen of threads about this very topic, included what's literally the most popular one in this section:
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=679595#Post679595At least until the one begging for more bikini armors won't top this as well.
You really don't need any specific classes. Take whatever companions you want, you can still do fine.
"Need" is overrated. This is not really about what's needed, as much as being about the enjoyment that comes from opening to an exponentially greater number of combinations.