Synonym of "at a time"

Is there a synonym of at a time? For example,

Only four controllers can be connected at a time.

Simultaneously and contemporaneously and are both viable options.

Concurrently would also be acceptable.

If you type in "at the same time" at the onelook.com reverse-dictionary, you may find the first seven entries (1. synchronize; 2. synchronous; 3. contemporary; 4. simultaneous; 5. concurrent; 6. contemporaneous; 7. coeval) relevant. Most of the higher-numbered entries that seem relevant are adjectival or adverbial forms redundant with those, except for 17. coincidental and 32. coexistent. Some obscure or less-relevant terms are 47. abreast; 52. monochronic; 94. tautochronous; 108. isochronous; 118. commorient (dying at the same time); 138. coextensive; 144. coetaneous; 150. connascent (born at the same time); 182. synchronoptic (seen at the same time by one person).