Close-up view of the AR-15 #000003,slip ring/two pice conic forearm in a #000001 receiver.

I suspect that they are two different rifles. I think it is quite possible that Armalite made #0004 with Pegasus logo and then when Colt started producing rifles they started their serial numbers at 0001 and built their own #0004 with a Rampant Colt logo. Just as Howa produced AR-180's with serial numbers S0001-S1000 and when Armalite started producing the rifles in Costa Mesa they started the serial numbers at S0001. Later when production at Howa resumed, serial numbers started at S10001.

There is one other possibility.
At the time Colt was retooling,ArmaLite was making the rifles as Colt was financing it.
The AR-15 #000004 was owned by Bobby Macdonald salesman for Cooper- Macdonald,the #000004 was used as a showcase pice.
Cooper- Macdonald used the AR-15 #000004,owned by Bobby Macdonald,in the early brochures for Colt-ArmaLite.
The #000004 may have been a transitional unit and it was fitted with diferent parts as the technology advanced,the #000004 is not just a random unit,it is because it was owned by Bobby Macdonald from Cooper- Macdonald,they were working the deal between Colt and ArmaLite and the showing the unit and making brochures for it as they were also trying to sell the AR-15 for Colt to get other markets.
Cooper- Macdonald was being paid by Colt and ArmaLite and when the deal was done Cooper- Macdonald got a royalty fee for helping with the deal, the #000004 may have started using a new two pice conic forearm whit a new slip ring in a #000002 type upper receiver but refitted with new parts as they got developed,the #000004 may have started as a in progress development prototype but in the end it was refitted with parts close to what would be the final look for the AR-15,it was used as a technology showcase.

The upper and lower receivers in the #000004 are a tell sign,the lower receiver is the early normal as the upper is a covered one but made to fit to what was then the normal lower receiver,later on the Ar-15 receivers had a section in the left side that went in the lower receiver and covered in the upper receiver.





In the Colt AR-15 brochure,the #000004 lower receiver is using a new upper receiver,a new front sight,a new two pice triangular forearm with slipring,and a new butstock,the stocks are new as they no longer have the exposed fiberglass,this layout is the final one but the new upper receiver layout (as a prototype) was made to fit to the geometry of the original #000004 lower receiver.
The #000004 was the linking piece between the development and final phase.
The photo above shows the last and final layout for the AR-15.

Soldiers of the Future (1950s America)

The history of the AR-15 starts not with the down scaled of the AR-10 but before.
It started with the AR-11 around 1957,ArmaLite needed a rifle platform to test and demonstrate the use of the .222 to the military.
At the time,1957,the AR-10 was being tested at the Springfield Armory,it self the AR-10 was not not yet a fully developed platform,ArmaLite choose to use the .222 on a more conventional platform so they based this new unit in  the M7,tested at the Springfield Armory in 1955,and designated it as the AR-11.
The ArmaLite AR-11 was a down scaled M7,it had a fiberglass stock/aluminium receiver/8 lug rotaty bolt head/conventional gas system.
Later on,as the production of the AR-10 shifted to Holland ( Artillerie- Inrichtengen),ArmaLite then focused on the development AR-15 around the .222.

Looking the pics over again. Kinda looks like these HGs,


are still not these:



But more likely these:



Maybe it is just the angle of the rifle and the grainyness of the pic, but the HGs in the middle photo still look like triangles to me. And this is from an article from 1959? I still like the proto HGs in the third pic, event if they are not triangles, they seem to be a transition from conic to triangle.

The ArmaLite AR-11 was also known as the "Stopette".
"Stopette" cames from a spray deodorant,the AR-11 (later AR-15) was intended to spray bullets.



ArmaLite AR-11 Stopette "POOF! there goes commie"

I guess you had to be there.  

I wander if the "AR-3" in this image isn't actually a AR-11 "Stopette".

Note that in the first image,it as a short capacity waffle magazine.

Complete AR-15/M16 Sourcebook image.

In the second image (Springfield Armory) it as a BAR type magazine



Note that the butstock pads are diferent.