can you post some data . oldbrewers formula (combined here with your iserror) works for me
formula =IFERROR(VLOOKUP(VALUE(LEFT(B2,4)),Sheet6!$A$2:$Z$10,2,FALSE),"** UNKNOWN SUPPLIER **")
main sheet
[TABLE="width: 246"]
<COLGROUP><COL style="WIDTH: 48pt" span=2 width=64><COL style="WIDTH: 150pt; mso-width-source: userset; mso-width-alt: 7314" width=200><TBODY>[TR]
[TD="width: 64, bgcolor: transparent"]name[/TD]
[TD="width: 64, bgcolor: transparent"]number[/TD]
[TD="width: 200, bgcolor: transparent"][/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]al[/TD]
[TD="bgcolor: transparent, align: right"]12345555[/TD]
[TD="bgcolor: transparent"]BigPhoneC[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]beth[/TD]
[TD="bgcolor: transparent, align: right"]24567890[/TD]
[TD="bgcolor: transparent"]AN Other Co[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]col[/TD]
[TD="bgcolor: transparent, align: right"]12346666[/TD]
[TD="bgcolor: transparent"]BigPhoneC[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]deb[/TD]
[TD="bgcolor: transparent, align: right"]88881111[/TD]
[TD="bgcolor: transparent"]Cheap Phones[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]ed[/TD]
[TD="bgcolor: transparent, align: right"]24569999[/TD]
[TD="bgcolor: transparent"]AN Other Co[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]fran[/TD]
[TD="bgcolor: transparent, align: right"]24689898[/TD]
[TD="bgcolor: transparent"]Rich Phone Co[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]gail[/TD]
[TD="bgcolor: transparent, align: right"]24568765[/TD]
[TD="bgcolor: transparent"]AN Other Co[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]harri[/TD]
[TD="bgcolor: transparent, align: right"]88881111[/TD]
[TD="bgcolor: transparent"]Cheap Phones[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]inga[/TD]
[TD="bgcolor: transparent, align: right"]24687654[/TD]
[TD="bgcolor: transparent"]Rich Phone Co[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]jon[/TD]
[TD="bgcolor: transparent, align: right"]12344321[/TD]
[TD="bgcolor: transparent"]BigPhoneC[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]kate[/TD]
[TD="bgcolor: transparent, align: right"]24687777[/TD]
[TD="bgcolor: transparent"]Rich Phone Co[/TD]
[/TR]
[TR]
[TD="bgcolor: transparent"]len[/TD]
[TD="bgcolor: transparent, align: right"]55557777[/TD]
[TD="bgcolor: transparent"] * UNKNOWN SUPPLIER*[/TD]
[/TR]
</TBODY>[/TABLE]
lookup sheet
[TABLE="width: 151"]
<TBODY>[TR]
[TD]code</SPAN>[/TD]
[TD]name</SPAN>[/TD]
[/TR]
[TR]
[TD="align: right"]1234</SPAN>[/TD]
[TD]BigPhoneC</SPAN>[/TD]
[/TR]
[TR]
[TD="align: right"]2468</SPAN>[/TD]
[TD]Rich Phone Co</SPAN>[/TD]
[/TR]
[TR]
[TD="align: right"]2456</SPAN>[/TD]
[TD]AN Other Co</SPAN>[/TD]
[/TR]
[TR]
[TD="align: right"]8888</SPAN>[/TD]
[TD]Cheap Phones</SPAN>[/TD]
[/TR]
</TBODY><COLGROUP><COL><COL></COLGROUP>[/TABLE]
[TABLE="width: 151"]
<TBODY>[TR]
[TD]code</SPAN>[/TD]
[TD]name</SPAN>[/TD]
[/TR]
[TR]
[TD="align: right"]1234</SPAN>[/TD]
[TD]BigPhoneC</SPAN>[/TD]
[/TR]
[TR]
[TD="align: right"]2468</SPAN>[/TD]
[TD]Rich Phone Co</SPAN>[/TD]
[/TR]
[TR]
[TD="align: right"]2456</SPAN>[/TD]
[TD]AN Other Co</SPAN>[/TD]
[/TR]
[TR]
[TD="align: right"]8888</SPAN>[/TD]
[TD]Cheap Phones</SPAN>[/TD]
[/TR]
</TBODY><COLGROUP><COL><COL></COLGROUP>[/TABLE]