7  letters Place 4 = 7P4 = 7*6*5*4 = 840.  Here they all are,
84 rows of 10 each, computer generated:

DECA, DECG, DECO, DECN, DEAC, DEAG, DEAO, DEAN, DEGC, DEGA,
DEGO, DEGN, DEOC, DEOA, DEOG, DEON, DENC, DENA, DENG, DENO,
DCEA, DCEG, DCEO, DCEN, DCAE, DCAG, DCAO, DCAN, DCGE, DCGA,
DCGO, DCGN, DCOE, DCOA, DCOG, DCON, DCNE, DCNA, DCNG, DCNO,
DAEC, DAEG, DAEO, DAEN, DACE, DACG, DACO, DACN, DAGE, DAGC,
DAGO, DAGN, DAOE, DAOC, DAOG, DAON, DANE, DANC, DANG, DANO,
DGEC, DGEA, DGEO, DGEN, DGCE, DGCA, DGCO, DGCN, DGAE, DGAC,
DGAO, DGAN, DGOE, DGOC, DGOA, DGON, DGNE, DGNC, DGNA, DGNO,
DOEC, DOEA, DOEG, DOEN, DOCE, DOCA, DOCG, DOCN, DOAE, DOAC,
DOAG, DOAN, DOGE, DOGC, DOGA, DOGN, DONE, DONC, DONA, DONG,
DNEC, DNEA, DNEG, DNEO, DNCE, DNCA, DNCG, DNCO, DNAE, DNAC,
DNAG, DNAO, DNGE, DNGC, DNGA, DNGO, DNOE, DNOC, DNOA, DNOG,
EDCA, EDCG, EDCO, EDCN, EDAC, EDAG, EDAO, EDAN, EDGC, EDGA,
EDGO, EDGN, EDOC, EDOA, EDOG, EDON, EDNC, EDNA, EDNG, EDNO,
ECDA, ECDG, ECDO, ECDN, ECAD, ECAG, ECAO, ECAN, ECGD, ECGA,
ECGO, ECGN, ECOD, ECOA, ECOG, ECON, ECND, ECNA, ECNG, ECNO,
EADC, EADG, EADO, EADN, EACD, EACG, EACO, EACN, EAGD, EAGC,
EAGO, EAGN, EAOD, EAOC, EAOG, EAON, EAND, EANC, EANG, EANO,
EGDC, EGDA, EGDO, EGDN, EGCD, EGCA, EGCO, EGCN, EGAD, EGAC,
EGAO, EGAN, EGOD, EGOC, EGOA, EGON, EGND, EGNC, EGNA, EGNO,
EODC, EODA, EODG, EODN, EOCD, EOCA, EOCG, EOCN, EOAD, EOAC,
EOAG, EOAN, EOGD, EOGC, EOGA, EOGN, EOND, EONC, EONA, EONG,
ENDC, ENDA, ENDG, ENDO, ENCD, ENCA, ENCG, ENCO, ENAD, ENAC,
ENAG, ENAO, ENGD, ENGC, ENGA, ENGO, ENOD, ENOC, ENOA, ENOG,
CDEA, CDEG, CDEO, CDEN, CDAE, CDAG, CDAO, CDAN, CDGE, CDGA,
CDGO, CDGN, CDOE, CDOA, CDOG, CDON, CDNE, CDNA, CDNG, CDNO,
CEDA, CEDG, CEDO, CEDN, CEAD, CEAG, CEAO, CEAN, CEGD, CEGA,
CEGO, CEGN, CEOD, CEOA, CEOG, CEON, CEND, CENA, CENG, CENO,
CADE, CADG, CADO, CADN, CAED, CAEG, CAEO, CAEN, CAGD, CAGE,
CAGO, CAGN, CAOD, CAOE, CAOG, CAON, CAND, CANE, CANG, CANO,
CGDE, CGDA, CGDO, CGDN, CGED, CGEA, CGEO, CGEN, CGAD, CGAE,
CGAO, CGAN, CGOD, CGOE, CGOA, CGON, CGND, CGNE, CGNA, CGNO,
CODE, CODA, CODG, CODN, COED, COEA, COEG, COEN, COAD, COAE,
COAG, COAN, COGD, COGE, COGA, COGN, COND, CONE, CONA, CONG,
CNDE, CNDA, CNDG, CNDO, CNED, CNEA, CNEG, CNEO, CNAD, CNAE,
CNAG, CNAO, CNGD, CNGE, CNGA, CNGO, CNOD, CNOE, CNOA, CNOG,
ADEC, ADEG, ADEO, ADEN, ADCE, ADCG, ADCO, ADCN, ADGE, ADGC,
ADGO, ADGN, ADOE, ADOC, ADOG, ADON, ADNE, ADNC, ADNG, ADNO,
AEDC, AEDG, AEDO, AEDN, AECD, AECG, AECO, AECN, AEGD, AEGC,
AEGO, AEGN, AEOD, AEOC, AEOG, AEON, AEND, AENC, AENG, AENO,
ACDE, ACDG, ACDO, ACDN, ACED, ACEG, ACEO, ACEN, ACGD, ACGE,
ACGO, ACGN, ACOD, ACOE, ACOG, ACON, ACND, ACNE, ACNG, ACNO,
AGDE, AGDC, AGDO, AGDN, AGED, AGEC, AGEO, AGEN, AGCD, AGCE,
AGCO, AGCN, AGOD, AGOE, AGOC, AGON, AGND, AGNE, AGNC, AGNO,
AODE, AODC, AODG, AODN, AOED, AOEC, AOEG, AOEN, AOCD, AOCE,
AOCG, AOCN, AOGD, AOGE, AOGC, AOGN, AOND, AONE, AONC, AONG,
ANDE, ANDC, ANDG, ANDO, ANED, ANEC, ANEG, ANEO, ANCD, ANCE,
ANCG, ANCO, ANGD, ANGE, ANGC, ANGO, ANOD, ANOE, ANOC, ANOG,
GDEC, GDEA, GDEO, GDEN, GDCE, GDCA, GDCO, GDCN, GDAE, GDAC,
GDAO, GDAN, GDOE, GDOC, GDOA, GDON, GDNE, GDNC, GDNA, GDNO,
GEDC, GEDA, GEDO, GEDN, GECD, GECA, GECO, GECN, GEAD, GEAC,
GEAO, GEAN, GEOD, GEOC, GEOA, GEON, GEND, GENC, GENA, GENO,
GCDE, GCDA, GCDO, GCDN, GCED, GCEA, GCEO, GCEN, GCAD, GCAE,
GCAO, GCAN, GCOD, GCOE, GCOA, GCON, GCND, GCNE, GCNA, GCNO,
GADE, GADC, GADO, GADN, GAED, GAEC, GAEO, GAEN, GACD, GACE,
GACO, GACN, GAOD, GAOE, GAOC, GAON, GAND, GANE, GANC, GANO,
GODE, GODC, GODA, GODN, GOED, GOEC, GOEA, GOEN, GOCD, GOCE,
GOCA, GOCN, GOAD, GOAE, GOAC, GOAN, GOND, GONE, GONC, GONA,
GNDE, GNDC, GNDA, GNDO, GNED, GNEC, GNEA, GNEO, GNCD, GNCE,
GNCA, GNCO, GNAD, GNAE, GNAC, GNAO, GNOD, GNOE, GNOC, GNOA,
ODEC, ODEA, ODEG, ODEN, ODCE, ODCA, ODCG, ODCN, ODAE, ODAC,
ODAG, ODAN, ODGE, ODGC, ODGA, ODGN, ODNE, ODNC, ODNA, ODNG,
OEDC, OEDA, OEDG, OEDN, OECD, OECA, OECG, OECN, OEAD, OEAC,
OEAG, OEAN, OEGD, OEGC, OEGA, OEGN, OEND, OENC, OENA, OENG,
OCDE, OCDA, OCDG, OCDN, OCED, OCEA, OCEG, OCEN, OCAD, OCAE,
OCAG, OCAN, OCGD, OCGE, OCGA, OCGN, OCND, OCNE, OCNA, OCNG,
OADE, OADC, OADG, OADN, OAED, OAEC, OAEG, OAEN, OACD, OACE,
OACG, OACN, OAGD, OAGE, OAGC, OAGN, OAND, OANE, OANC, OANG,
OGDE, OGDC, OGDA, OGDN, OGED, OGEC, OGEA, OGEN, OGCD, OGCE,
OGCA, OGCN, OGAD, OGAE, OGAC, OGAN, OGND, OGNE, OGNC, OGNA,
ONDE, ONDC, ONDA, ONDG, ONED, ONEC, ONEA, ONEG, ONCD, ONCE,
ONCA, ONCG, ONAD, ONAE, ONAC, ONAG, ONGD, ONGE, ONGC, ONGA,
NDEC, NDEA, NDEG, NDEO, NDCE, NDCA, NDCG, NDCO, NDAE, NDAC,
NDAG, NDAO, NDGE, NDGC, NDGA, NDGO, NDOE, NDOC, NDOA, NDOG,
NEDC, NEDA, NEDG, NEDO, NECD, NECA, NECG, NECO, NEAD, NEAC,
NEAG, NEAO, NEGD, NEGC, NEGA, NEGO, NEOD, NEOC, NEOA, NEOG,
NCDE, NCDA, NCDG, NCDO, NCED, NCEA, NCEG, NCEO, NCAD, NCAE,
NCAG, NCAO, NCGD, NCGE, NCGA, NCGO, NCOD, NCOE, NCOA, NCOG,
NADE, NADC, NADG, NADO, NAED, NAEC, NAEG, NAEO, NACD, NACE,
NACG, NACO, NAGD, NAGE, NAGC, NAGO, NAOD, NAOE, NAOC, NAOG,
NGDE, NGDC, NGDA, NGDO, NGED, NGEC, NGEA, NGEO, NGCD, NGCE,
NGCA, NGCO, NGAD, NGAE, NGAC, NGAO, NGOD, NGOE, NGOC, NGOA,
NODE, NODC, NODA, NODG, NOED, NOEC, NOEA, NOEG, NOCD, NOCE,
NOCA, NOCG, NOAD, NOAE, NOAC, NOAG, NOGD, NOGE, NOGC, NOGA.

---------------------

Some of them are REAL WORDS! J:

ACED, ACNE, AGED, CAGE, CANE, CODA, CODE, COED, CONE, DEAN, 
DONE, GOAD, GONE, NODE, ONCE  

Edwin