Is the bowden cable too tight? Have you tried slackening it? 4th gear requires the most fluid to engage the correct brake bands and clutches. If you have low fluid, low operating pressure, or excessive internal wear on the transmission, 4th gear is usually the first to show issues.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]