AC/DC 4W vs 2A GSM transmission bursts
2A bursts are 577 µs every 4.6 ms


The power for those bursts is equivalent to about 1.25 watts\$^1\$ - if the 4.1V supply were 100% efficient at taking energy from the 5V traco then the power needed would be 1.028 watts but because you are using a linear voltage regulator to create the 4.1 volts then it's 1.25 watts from the traco.

The question is, from your 4 watt traco supply, is the remaining 2.75 watts enough to power the circuits that don't take the 2 A pulse. If it is then that's OK you don't necessarily need to change the traco and you can provide the "surge" power with a large capacitor, preferably directly on the power feeds to the SIM900.

How big? 2 amps at 4.1 volts for 0.577 ms is an energy of 4.73 milli joules so rearranging the well-know energy-capacitance-voltage formula gives you: -

\$C = 2\dfrac{energy}{V^2}= 2\dfrac{4.73 mJ}{16.81}\$ = 563 uF BUT this needs to but at least 5 times bigger so the voltage at the terminals doesn't droop - consider that it needs to be about 3000 uF and use a low ESR type.



\$^1\$ average power is 5 volts x 2 amps x 0.577 / 4.6 = 1.25 watts.