I'm using a BL652 and so far I've been able to send single packages of 20 bytes per connection interval using a custom service. How can I send multiple packages (i.e. 4)?

In order to configure the BL652 to send multiple packages you will need to ensure you have upgraded the module to the 28.6.2.0 firmware version (or newer), which is available from the Software Downloads section of the BL652 Product Page. 

Please reference the section that cover Higher Bandwidth Configuration on page seven of the BL652 Data Length Extension Application Note.

Excerpt from the Application Note: 

Higher Bandwidth Configuration With the release of v28.6.2.0 of the BL652 firmware, the higher bandwidth configuration feature was also added. This feature enables the BL652 to send up to 6 packets per connection interval, therefore significantly improving the throughput (the default value on the BL652 is 3 packets per connection interval). Higher bandwidth configuration can be enabled using AT+CFG 214 1 before startup or using the smartBASIC function NvCfgKeySet (214, 1). In both cases, the board should be reset (using ATZ or Reset(0) ) so that the new value takes effect.

Additional information regarding configuration using AT+CFG 214 can be found on page 19 of the User Guide: User Guide - BL652 smartBASIC Extensions - v28.6.2.0  [Per User Guide] AT+CFG 214 0: Medium bandwidth (3 packets per connection interval) is used on all connections. 1: High bandwidth (6 packets per connection interval) is used on the FIRST connection. When multiple devices are connected, only the first device will have the high bandwidth, the additional connections will have medium bandwidth. Note: When high bandwidth is used, the maximum number of connections that a device can have are reduced from 8 to 6.