Alternatives to replace R404A for refrigerated transport
With about 5 million refrigerated vehicles operating worldwide, there is active research and development focusing on identifying alternatives to R404A, which is traditionally used in truck and trailer refrigerating units. R452A, R463A and CO2 are presented here as a few interesting alternative refrigerants.
According to an IIR informatory note, there are about 5 million refrigerated vehicles operating worlwide.  The choice of refrigerant in the refrigeration systems of transport vehicles is an important issue, especially in Europe, in compliance with the EU F-gas Regulation No 517/2014.
Since January 2020, the use of F-gases with a GWP of 2500 or more, in service or maintenance of refrigeration equipment with a charge size of 40 tonnes CO2 eq. or more has been prohibited. Traditionally used in truck and trailer refrigeration units, R404A is affected by the maintenance/service ban.  According to the 2018 RTOC Assessment Report, R404A has a GWP of 4200. 
Refrigerated transport is one of the most challenging field of applications for vapour compression cycles in the cold chain. The refrigeration unit must be small, which often inhibits the application of complex solutions. Compared with stationary refrigeration systems, refrigeration units in vehicles operate in harsher environment and usually present a lower Coefficient of Performance (COP). 
R452A as an alternative to R404A
R452A is a drop-in alternative, currently the most used to replace R404A. R452A is a ternary zeotropic mixture of R1234yf, R32 and R125 (30/11/59% by mass). It is an HFO/HFC mixture with a GWP of 2100. R452A has similar refrigerating capacity, reliability and refrigerant charge as R404A. R452A is suitable for use in refrigerated transport because of its non-flammability and low compressor discharge temperatures. The similarity of properties between R452A and R404A gives customers the opportunity to retrofit their existing fleets and operate on R452A. 
In the refrigeration systems used for food transportation, a study found that CO2 equivalent emissions were reduced by 5-15% with R452A.  The main limitation of R452A is its GWP, which is only 45% lower than that of R404A.
Carbon dioxide (CO2) as an alternative to R404A
CO2 (R744) is a promising substitute, thanks to its Global Warming Potential (GWP) of 1. However, R744 cannot be used in the same way as other industrial refrigerants due to its low critical temperature of 31°C. It needs to be either coupled with a higher temperature refrigerant in a cascade system or used in a transcritical system.
In order to improve system performance in a hot ambient environment, a recent study presents a model for a refrigeration system designed to extract heat from the cargo space of a refrigerated vehicle and reject it to the external environment.  The authors simulated the performance of a CO2 transcritical system under three configurations: the standard back-pressure with low pressure receiver layout and two arrangements integrating a two-phase ejector.
When the system is operating in a hot climate, simulation results show that the ejector cycle configuration yields a maximum COP increase of 15.9% (compared to the traditional configuration), at 42 °C ambient temperature and -5 °C internal space temperature. The use of an auxiliary evaporator can extend the operating range of the ejector to lower values of ambient temperature. Compared with the traditional configuration, a maximum COP improvement of 21.0% was achieved at 25°C ambient temperature and 5°C internal cargo space temperature.
R463A as an alternative to R404A 
R463A is a non-flammable hydrofluoroolefin (HFO) refrigerant mixture that was developed to be retrofitted to R404A. R463A is an azeotropic mixture of R32 (36%), R125 (30%), R134a (14%), R1234yf (14%), and CO2 (R744) (6%). The presence of HFO 1234yf in its composition contributes to lowering the GWP. According to research, RA63A has a GWP of 1494, which is 63% lower than R404A.
Compared with R404A, R463A has a normal boiling point 23% higher, as well as a higher refrigerating capacity. The critical pressure and temperature of R463A are higher than those of R404A. It can be used in a high-ambient-temperature environment and has higher refrigerating and heat-rejection effect. Under low-temperature applications, the cooling coefficient of performance (COPc) of R463A is 10% higher than that of R404A.
 38th Note on Refrigeration Technologies: The Role of Refrigeration in the Global Economy (2019). https://iifiir.org/en/fridoc/142028
 2018 Assessment Report of the Refrigeration, Air Conditioning and Heat Pumps Technical Options Committee (RTOC). https://ozone.unep.org/sites/default/files/2019-04/RTOC-assessment-report-2018_0.pdf
 Artuso, Paolo, et al. "Modelling the performance of a new cooling unit for refrigerated transport using carbon dioxide as the refrigerant." International Journal of Refrigeration (2020). https://iifiir.org/en/fridoc/142305
 Heredia-Aricapa, Y., et al. "Overview of low GWP mixtures for the replacement of HFC refrigerants: R134a, R404A and R410A." International Journal of Refrigeration 111 (2020): 113-123. https://iifiir.org/fr/fridoc/142084
 Saengsikhiao P, Taweekun J, Maliwan K, Sae-ung S, Theppaya T. Investigation and Analysis of R463A as an Alternative Refrigerant to R404A with Lower Global Warming Potential. Energies. 2020; 13(6):1514. https://doi.org/10.3390/en13061514