“I’ve noticed that there are several verses missing in the NIV Bible (e.g., Mk. 16:9-20; Jn. 5:4; Acts 8:37; I Jn. 5:7), which bothers me. So, should I only use the King James Version (KJV)?”

The answer is “No:”

Even though the KJV is a great translation, the newer translations (like the NIV) are also good ones; they simply use older manuscripts that were unavailable when the King James’ translators compiled the KJV in 1611.

The KJV translators used a Greek manuscript known as the Textus Receptus. But, since that time, many other Biblical manuscripts have been discovered that predate the one used in 1611. And Biblical scholars and textual critics discovered there were some differences between the one used for the KJV and earlier manuscripts.

That’s why the newer translations place the “missing verses” in footnotes or brackets because it appears they were added later. For example, Jn. 5:4 is included in the KJV, but in the New King James Version (NKJV) there’s a footnote stating that it wasn’t found in many Greek texts.

Since that verse wasn’t in the oldest manuscripts, later scribes added it in the text since v.7 talks about the water being stirred. It was added for clarity and doesn’t take anything away from the story of Jesus healing the man who’d been at the Pool of Bethesda for 38 years.

The same was true with the other verses cited in the question. But, again, none of these “missing verses,” footnotes or words/phrases in brackets have any impact upon the truths in God’s Word. That’s why the most important Truths. . .about Jesus being the Only Way of Salvation. . .the doctrines of Heaven and hell. . .sin and Redemption. . .the Nature and Character of God, etc. . . are preserved in all translations through the work of the Holy Spirit.

Yes, regardless of the type of Biblical translation you have (KJV, NIV, RSV, ESV, ASV, etc.), rest in the assurance that the Holy Spirit has made sure what’s most important is included in all of them (II Tim. 3:16-17).