Hi Igor,

this LCD-controllers are exacly the same, only different manufacturers:

HD44780 (Hitachi)
KS0066 (Samsung)
SED1278 (Epson)

So if your code works with two line displays, the only I can think of is your 4-lines x 20 display contains TWO controllers. Although this is not common. Look carefully at your display to be sure how many controllers there are on it (not: support chips !!)
How many connections do you have? if there are more than 14, it can be dual controller display, in that case you have two EN (enable) lines, and have to control them seperately.

Commomly, for 4 x 20 displays, they should work the same like two line displays. From the programmer's point of view, the tirth line is the is the continuation of the first line. The second line starts at the same adres like the second line of a two lines display. The fourth line is the continuation of the second line. With this information you should be able to solve your problem.

Henk