4. Is there a ban on corporate donations to candidates?

Source 1: Article 64, paragraph 1, Political Parties Act 2013.  

Artículo 64 inciso 1°. – Las contribuciones privadas solo pueden acreditarse a favor de los partidos políticos.

Article 64, paragraph 1. – Private contributions can only be credited to political parties.

Source 2: Article 10, paragraph 1, Provisions for the Nomination of Non-Party Candidates in
Legislative Elections. 2010

Artículo 10 inciso 1°.- El candidato no partidario podrá recibir donaciones de fuentes privadas, para lo cual abrirá una cuenta bancaria única a su nombre o del Grupo de Apoyo que lo respalda.

Article 10, paragraph 1. – Non-party candidates may receive donations from private sources, for which purpose they shall open a single bank account in their name or in the name of the Support Group backing them.

Ley de Partidos: https://www.asamblea.gob.sv/sites/default/files/documents/decretos/2E597B83-D1C9-4C91-A9C9-6EC8A44FBA0F.pdf  
Regulación candidaturas no partidarias: https://www.asamblea.gob.sv/sites/default/files/documents/decretos/45E06ECC-27EC-49EC-B5D5-5D29814BAD32.pdf   

There is no explicit prohibition on corporate donations to candidates. For non-party candidates, Article 10(1) of the 2010 Decree allows them to receive donations from private sources without specifying any restrictions on legal entities. For party candidates, this does not apply, since only political parties are entitled to receive private contributions under Article 64 of the Political Parties Act (2013), which does not prohibit donations from corporate sources.
In fact, by not regulating the financing of party candidates, the law creates a loophole that is exploited in practice by candidates to raise funds without being accountable to their parties or the electoral authority for the origin and destination of the funds received.