On ArchLinux I have installed Ark.
Output from the package manager:

local/kdeutils-ark 15.04.3-1 (kde-applications kdeutils)
Archiving Tool

Ark is also in the list of KDE 5 applications so I assumed it will behave like one but when it has old look and feel
and when I open the about dialog I see:

Version 2.19
Using KDE Development Platform 4.14.10

Now I do not understand this especially if Konsole app installed on the same system behaves as expected.

rsupremo wrote:Ark is also in the list of KDE 5 applications so I assumed it will behave like one but when it has old look and feel

No, it isn't.
Ark from KDE Applications 15.04.3 is still KDE4 based.

A KF5 version has not been released yet, but will probably be part of 15.08 in August.

Now I do not understand this especially if Konsole app installed on the same system behaves as expected.

Konsole is indeed KF5 based since the 15.04 release.

OTOH, if you set a "look and feel" in Plasma5's systemsettings5 it should also be applied to KDE4 applications.