Easy strumming was probably the market reason for the creation of the so called plectrum banjo. For strumming you don't need a pick for each finger, and because of the metal strings the most natural choice is a flat pick. And, with a flat pick you can also do soloing or combine melodic lines with selectic pluking of two or more strings (a kind of guitar style flat picking).
However the name "plectrum banjo" is deceptive as you can still apply a number of other techniques with it.
There's no written rule that says you mustn't use thumb and finger picks with a 4 string banjo, and apply some kind of finger picking technique (you would have to use patterns more similar to guitar's finger picking, than to banjo's, as you don't have the drone 5th string).
Or, for that matter, that you can't play with any pick at all, like with a folk guitar, or try a kind of clawhammer style (although without the 5th string the result will not be equivalent to true clawhammer).
Dom Flemons (of Carolina Chocolate Drops fame) has an amazing 4 string banjo mixed technique, using a single thumb pick to alternate strumming with soloing (see for example here)