"Actually, even if the computer energizes the main relay, the fuse or wiring could still be bad."
• Not so. The "computer" cannot energize the main relay. In fact it's the Main relay that supplies operational* voltage to the ECU/Computer.
* Not to be confused with preliminary voltage from the Ignition switch.

"The ECU has an alternate supply from the ignition switch."
• Not so. The voltage from the ignition switch (to ECU 35) is merely a "wake up", and is not an "alternative" to the operating voltage sent to ECU 9 from the Main relay, voltage which originates at the inline fuse.

However, the ECU voltage is not the issue here. The poster questions (and rightly so) the lack of pump voltage at fuse 4, thinking it should be there when the "fuel pump relay" clicks, apparently unaware that the "click of a relay closing" was the Main relay, not the Fuel relay. Or even that there are actually two relays involved.

"Since the relay was replaced, the fused main supply could be defective."
• True, but not germane to this problem as I interpreted the poster's comments. See below.

"EDIT: The poster never stated he felt the relay click!"
• Maybe not in so many words, but his opening paragraph showed a working knowledge of the fuse 4 circuit, and his closing paragraph (abbreviated below) clearly implied that the "click of relay closing" did not "energize fuse panel #4" (as expected, based on the click).

". . . should't I be able to feel the click of relay closing thus completing elec. circuit, if yes, then shouldn't this action energize fuse panel#4?"


--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.