  Four constructions for Ferrers diagram rank-metric (FDRM) codes are
presented. The first one makes use of a characterization on generator matrices
of a class of systematic maximum rank distance codes. By introducing restricted
Gabidulin codes, the second construction is presented, which unifies many known
constructions for FDRM codes. The third and fourth constructions are based on
two different ways to represent elements of a finite field $\mathbb F_{q^m}$
(vector representation and matrix representation). Each of these constructions
produces optimal codes with different diagrams and parameters.
