No, they are only similar in the "Silent Hunter" part of their names. "Silent Hunter 4: Wolves of the Pacific" was released as Pacific-centered initially, and started at v1.0.xxx (I have an original disk somewhere around here...
) and it was basically unplayable. Updates brought it up through various v1.1x, v1.2x, v1.3x, v1.4x until finally the last update of v1.5, in which they included a German Pacific campaign in the game, and they charged people money for the "upgrade". As per usual gaming industry "update" cycle, they would fix two issues and break one with each update - kind of like Windows does to this day to their OS... You might also see v1.5 of the game referred to as the "Gold Edition".
Anyway, Silent Hunter 5 was a return to the Battle of the Atlantic German U-Boats, very similar to SH3 in that regard, but they partially changed their "asset" structure, moving away from obj based files to the "Granny" gr2 file structure, and changed other things about the game. The Type VII is the only player boat, and it is next to impossible to add more U-Boats. Folks have tried, but time passes and they still have not succeeded. Ubisoft in the meantime, moved on after one update to SH5 v1.2 (also referred to as "Gold Edition", not to confuse the user...
) and came out with an arcade version of the game, only playable online in a browser, and basically dropped any further "improvements" to SH5. So it is less "developed" than SH4, and only by the determination of modders, playable.
SH5 has really nice graphics, better than SH4 by a long shot in most regards, similarly to SH4 graphics are much better than SH3's. However, as for submarine simulations, the SH3 environment will offer the player more of a claustrophobic "feel" in the game, which might have been spoiled in SH4 by its better graphics, and larger display ( opinion )... The Ubisoft games all find their "roots" in the SSI / Aegis original DOS-based Silent Hunter game from 1996, which Ubisoft had purchased during SH2 development.