In the interim you can update:
/core/ServiceProviders/MinphpBridge.php
At the bottom of the file is a set of TTLs:
$ttls = [
'ttl' => 1800, // 30 mins
'cookie_ttl' => 604800, // 7 days
];
Change the 'ttl' value to the number of seconds a session should last, e.g.:
$ttls = [
'ttl' => 14400, // 4 hours
'cookie_ttl' => 604800, // 7 days
];
Making these configurable from the config file should occur in v4.2.