I haven’t been having any problems with v3 that seem related to the introduction of v4. Check your console log to see what exception is being thrown; if necessary, add a try/catch which prints the exception to the log before rethrowing it.
The main problem I’ve had with lambdas has been getting them to respond quickly enough not to time out. Adjusting the timeout on the lambda itself helps but really shouldn’t be necessary. The biggest problem in my code appears to be happening when the lambda process is being spun up (for the first time, or after having been terminated because it was idle). It’s rumored that doing a Javascript compression (mild obfuscation, turning everything into single-character symbols) may help; eliminating unused libraries may also help. And I may need to change a just-in-time data retrieval to run as a separate scheduled process.
(May have just found my problem: Update loop confusion between local time and UTC. I knew that was going to be an issue and tried to cheat. Self-inflicted wounds.)