You have to remember that during Project Mercury, not only crewed spaceflight was in its infancy, but spaceflight itself was.
The Mercury-Redstone rocket could not achieve orbit, it wasn't powerful enough. Only Mercury-Atlas could, and the first crewed Mercury-Atlas flights was Glenns and thus orbital. So the answer is that they went orbital as soon as the LV allowed (and it took five uncrewed MA tests before they put a human in that stack).
The goals of MR-4 are described as following:
The main objective was to corroborate the man-in-space concept. The main configuration differences between the MR-3 spacecraft was the addition of a large viewing window and an explosively actuated side hatch.
The addition of the large viewing window was a result of a change requested by Mercury astronauts. This window allowed the astronauts to have a greater viewing area than the original side port windows. The field of view was 30 degrees in the horizontal plane and 33 degrees in the vertical. The window is composed of an outer panel of 0.35-inch thick Vycor glass and a 3-layer inner panel.
The explosively actuated side hatch was used for the first time on the MR-4 flight.
Flight successful but the spacecraft was lost during the post landing recovery period as a result of premature actuation of the explosively actuated side egress hatch.
Testing the new hatch before undertaking the next truly big step seems reasonable. The hatch blew early, so clearly the testing was warranted. You wouldn't want the hatch to blow during your orbit.
Another change from MR-3 to MR-4 was the addition of dampening material:
To reduce these vibrations, additional dampening material was added to the instrument compartment prior to the remaining flight.
Another facet is revealed by looking at the timeline.
- MA-1 took place July 29, 1960
- MA-2 took place February 21, 1961
- MA-3 took place April 25, 1961
- MR-3 took place May 5, 1961
- MR-4 took place July 21, 1961
- MA-4 took place September 13, 1961
- MA-5 took place November 29, 1961
- MA-6 finally took place February 20, 1962
By the time the suborbital flights were done, the Mercury-Atlas was not ready, making an orbital mission impossible at that time.