Can I connect multiple outputs into a single output?
The answer is NO:
It may be possible in theory but it may also create a direct short. In either case it would violate code (NEC).
If you have 4 receptacles on 1 120v circuit the maximum that could be used would be 20 amps,,, if wired with #12 wire and a 20 amp breaker.
As stated above this would be a code violation.
installing a 20 amp receptacle would allow for 20 amps and this would be the maximum current allowed by code.
If there are 2 circuits they are usually a Multiwire branch circuit and 1set is on L1 breaker the other on L2 combining them is a direct short.
There is no legal way to combine multiple standard 120v receptacles to provide more current than 20 amps.
Edit — added for further clarification:
Not just for code reason but electrical short circuits.
You cannot combine a receptacle that is part of a multiwire branch circuit OR receptacles that are fed from odd &amp; even breakers. Yes they are 120v
But combining ether of these 2 independently fed and legal circuits creates a dead short on the service.
They should be protected at no more than 20 amps but still may trip a main breaker and the branch circuits. independently wired they are code compliant but combining them as the OP wants to do can create this hazard.
See NEC 210.4 and for pictures exhibits 210.1 &amp; 210.2
Last if you had 2 different breakers on L1 let’s say breaker number 1 &amp; 3 these would be on the same leg and in theory you could get 40 amps.
However this would be another code violation as the smallest wires that can be paralleled is #1
So again the answer is NO.