R230 have 2 type of chassis. 2 bay chassis with 2 cables hard drive slot and 4 bay chassis with 4 hot plug hard drive slot. You can use 2 bay chassis for 4 hard drive as it is not compatible. You can refer below post with more details [URL]#M70665 
Hello all, I have recently acquired a PowerEdge r230.  I noticed it only has two hard drive slots.  Is it possible to add two new drives to this model and bring the total number of drives to four?  Or am I stuck with only two?  Thanks for any advice. Dave C
r230 convert from 2bay to 4bay