ありがとうございました。
事態は進展しました。

しかしプラグインのインストールは始まったのですが、最後の表示がKilledでしたので、どうも途中で止まってしまったようです。
管理画面のインストール済みプラグインの一覧にも表示されません。

ちなみにコンソール表示は下記の内容でした。
なお、このレンタルサーバーはシステムログを見ることができませんが、
これはサーバーの負荷が大きすぎたということなのでしょうか？

何を調べたらよいのか教えていただけないでしょうか？

よろしくお願いします。

php bin/console eccube:composer:require ec-cube/RelatedProduct4
<warning>Deprecation warning: require.ec-cube/ProductDisplayRank4 is invalid, it should not contain uppercase characters. Please use ec-cube/productdisplayrank4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
<warning>Deprecation warning: require.ec-cube/ProductReview4 is invalid, it should not contain uppercase characters. Please use ec-cube/productreview4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
<warning>Deprecation warning: require.ec-cube/Recommend4 is invalid, it should not contain uppercase characters. Please use ec-cube/recommend4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
<warning>Deprecation warning: require.ec-cube/RelatedProduct4 is invalid, it should not contain uppercase characters. Please use ec-cube/relatedproduct4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
<warning>Deprecation warning: require.ec-cube/Securitychecker4 is invalid, it should not contain uppercase characters. Please use ec-cube/securitychecker4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
<warning>You are running composer with xdebug enabled. This has a major impact on runtime performance. See https://getcomposer.org/xdebug</warning>
[11.3MiB/1.29s] <warning>Warning from https://repo.packagist.org: You are using an outdated version of Composer. Composer 2.0 is now available and you should upgrade. See https://getcomposer.org/2</warning>
[184.2MiB/8.46s] Using version ^4.0 for ec-cube/RelatedProduct4
[184.2MiB/8.46s] ./composer.json has been updated
[184.5MiB/8.80s] <warning>Deprecation warning: require.ec-cube/ProductDisplayRank4 is invalid, it should not contain uppercase characters. Please use ec-cube/productdisplayrank4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
[184.5MiB/8.80s] <warning>Deprecation warning: require.ec-cube/ProductReview4 is invalid, it should not contain uppercase characters. Please use ec-cube/productreview4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
[184.5MiB/8.80s] <warning>Deprecation warning: require.ec-cube/Recommend4 is invalid, it should not contain uppercase characters. Please use ec-cube/recommend4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
[184.5MiB/8.80s] <warning>Deprecation warning: require.ec-cube/RelatedProduct4 is invalid, it should not contain uppercase characters. Please use ec-cube/relatedproduct4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
[184.5MiB/8.80s] <warning>Deprecation warning: require.ec-cube/Securitychecker4 is invalid, it should not contain uppercase characters. Please use ec-cube/securitychecker4 instead. Make sure you fix this as Composer 2.0 will error.</warning>
[187.8MiB/9.05s] Loading composer repositories with package information
[189.1MiB/10.22s] <warning>Warning from https://repo.packagist.org: You are using an outdated version of Composer. Composer 2.0 is now available and you should upgrade. See https://getcomposer.org/2</warning>
[189.2MiB/10.23s] Updating dependencies (including require-dev)
Killed